[FEATURE] Add json function to cast nested to json object

[acarbonetto]

Is your feature request related to a problem?

As part of the RFC to add JSON functions, the json() function would be useful to cast a nested object as a JSON_OBJECT using the json() function or cast(nested to json) syntax.

• META RFC: [META]Add PPL JSON extended functions support #3027
• PPL RFC: [RFC] Add PPL JSON extended functions support #3028
• Related Spark PPL PR: Support PPL JSON functions: construction and extraction opensearch-spark#780

What solution would you like?

• Add the json(nested) function that constructs a JSON_OBJECT from a nested object
• Add the cast(nested to json) casting syntax to cast a nested object to a JSON_OBJECT
• Update documentation and mapping

### `JSON`

**Description**

`json(value)` Unnests a NESTED object and casts it into a JSON object. 

**Argument type:** NESTED OBJECT

**Return type:** JSON OBJECT

Example:

    os> source=people | eval unnested_json = json(nested) | fields unnested_json
    fetched rows / total rows = 1/1
    +---------------------------------+
    | unnested_json                      |
    +---------------------------------+
    | {'name': 'Timmy', 'location': 'Seattle'   |
    +---------------------------------+

JSON Cast:

Example:

    os> source=people | eval unnested_json = cast(nested to json) | fields unnested_json
    fetched rows / total rows = 1/1
    +---------------------------------+
    | unnested_json                      |
    +---------------------------------+
    | {'name': 'Timmy', 'location': 'Seattle'   |
    +---------------------------------+

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