[FEATURE] Add json_array_length function to PPL

[acarbonetto]

Is your feature request related to a problem?

As part of the RFC to add JSON functions, the json_array function would be useful to construct json objects with arrays.

• META RFC: [META]Add PPL JSON extended functions support #3027
• PPL RFC: [RFC] Add PPL JSON extended functions support #3028
• Related Spark PPL PR: [followup] Refactor JSON function and add TO_JSON_STRING, ARRAY_LENGHT functions opensearch-spark#870

What solution would you like?

### `JSON_ARRAY_LENGTH`

**Description**

`json_array_length(doc[, path])` Returns the number of elements in array at path in the JSON document. By default, path is the root of the document. Returns NULL if the element at path is not an array. 

**Argument type:** JSON_OBJECT, STRING

**Return type:** INTEGER | NULL

Example:

    os> source=people | eval `length1` = json_array_length(json('[1,2,3,4]'), '{}'), `length2` = json_array_length(json('[1,2,3,{"f1":1,"f2":[5,6]},4]', '{}[3].f2'), `not_array` = json_array_length(json('{"key": 1}', '{}')
    fetched rows / total rows = 1/1
    +-----------+-----------+-------------+
    | lenght1   | lenght2   | not_array   |
    +-----------+-----------+-------------+
    | 4         | 2         | null        |
    +-----------+-----------+-------------+

What alternatives have you considered?

N/A

Do you have any additional context?

opensearch-project/opensearch-spark#870 - PR to add feature to opensearch-spark PPL