[FEATURE] Add json_append and json_extend functions to PPL

[acarbonetto]

Is your feature request related to a problem?

As part of the RFC to add JSON functions, the json_append and json_extend functions allow users to manipulate the json document by adding elements to a json array. json_extend flattens arrays before appending the values to the json document array.

• META RFC: [META]Add PPL JSON extended functions support #3027
• PPL RFC: [RFC] Add PPL JSON extended functions support #3028
• Related Spark PPL PR: PPL support json_delete, append functions opensearch-spark#971

What solution would you like?

### `JSON_APPEND`

**Description**

`json_append(doc, [path_key, list of values to add]...)`  appends values to end of an array at path within the json document. Return the updated json object. 

**Argument type:** JSON_OBJECT, List<STRING>

**Return type:** JSON_OBJECT

**Note**
Append adds the value to the end of the existing array with the following cases:
  - path is an object value - append is ignored and the value is returned
  - path is an existing array - the value are added to the array's tail
  - path not found - the value are added to the root of the json tree

`json_append` differs from `json_extend` in that it does not flatten arrays before appending them. 

Example:

    os> source=people | eval append = json_append(json('{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}'),'student', json('{"name":"Tommy","rank":5}')) | head 1 | fields append
    fetched rows / total rows = 1/1
    +-----------------------------------------------------------------------------------------------------------------------------------+
    | append                                                                                                                            |
    +-----------------------------------------------------------------------------------------------------------------------------------+
    |{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2},{"name":"Tommy","rank":5}]}                     |
    +-----------------------------------------------------------------------------------------------------------------------------------+

    os> source=people | eval append = json_append(json('{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}`,'teacher', json_array('Tom', 'Walt')) | head 1 | fields append
    fetched rows / total rows = 1/1
    +-----------------------------------------------------------------------------------------------------------------------------------+
    | append                                                                                                                            |
    +-----------------------------------------------------------------------------------------------------------------------------------+
    |{"teacher":["Alice",["Tom","Walt"]],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}                                 |
    +-----------------------------------------------------------------------------------------------------------------------------------+


    os> source=people | eval append = json_append(`{"school":{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}}`, 'school.teacher', 'Tom', 'school.teacher', 'Walt') | head 1 | fields append
    fetched rows / total rows = 1/1
    +-------------------------------------------------------------------------------------------------------------------------+
    | append                                                                                                                  |
    +-------------------------------------------------------------------------------------------------------------------------+
    |{"school":{"teacher":["Alice","Tom","Walt"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}}            |
    +-------------------------------------------------------------------------------------------------------------------------+

### `JSON_EXTEND`

**Description**

`json_extend(doc, [path_key, list of values to add]...)`  extends an array at path within the json document. Return the updated json document. 

**Argument type:** JSON_OBJECT, List<STRING>

**Return type:** JSON_OBJECT

**Note**
Append adds the value to the end of the existing array with the following cases:
  - path is an object value - append is ignored and the value is returned
  - path is an existing array - the value are added to the array's tail
  - path not found - the value are added to the root of the json tree

`json_extend` differs from `json_append` in that it flattens arrays before appending them the json document arrays. 

Example:

    os> source=people | eval extend = json_extend(json('{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}'),'student', json('{"name":"Tommy","rank":5}')) | head 1 | fields append
    fetched rows / total rows = 1/1
    +-----------------------------------------------------------------------------------------------------------------------------------+
    | extend                                                                                                                            |
    +-----------------------------------------------------------------------------------------------------------------------------------+
    |{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2},{"name":"Tommy","rank":5}]}                     |
    +-----------------------------------------------------------------------------------------------------------------------------------+

    os> source=people | eval extend = json_extend(json('{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}`,'teacher', json_array('Tom', 'Walt')) | head 1 | fields append
    fetched rows / total rows = 1/1
    +-----------------------------------------------------------------------------------------------------------------------------------+
    | extend                                                                                                                            |
    +-----------------------------------------------------------------------------------------------------------------------------------+
    |{"teacher":["Alice","Tom","Walt"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}                                 |
    +-----------------------------------------------------------------------------------------------------------------------------------+

    os> source=people | eval extend = json_extend(`{"school":{"teacher":["Alice"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}}`, 'school.teacher', 'Tom', 'school.teacher', 'Walt') | head 1 | fields append
    fetched rows / total rows = 1/1
    +-------------------------------------------------------------------------------------------------------------------------+
    | extend                                                                                                                  |
    +-------------------------------------------------------------------------------------------------------------------------+
    |{"school":{"teacher":["Alice","Tom","Walt"],"student":[{"name":"Bob","rank":1},{"name":"Charlie","rank":2}]}}            |
    +-------------------------------------------------------------------------------------------------------------------------+

What alternatives have you considered?

N/A

Do you have any additional context?

opensearch-project/opensearch-spark#780 - PR to add feature to opensearch-spark PPL