[FEATURE] Add to_json_string function to PPL

[acarbonetto]

Is your feature request related to a problem?

As part of the RFC to add JSON functions, the to_json_string function would be useful to convert json objects into json-encoded strings.

• 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?

### `TO_JSON_STRING`

**Description**

`to_json_string(doc)` Returns a JSON string representing the given json_document object.

**Argument type:** JSON_OBJECT

**Return type:** STRING

Example:

    os> source=people | eval `json_string` = to_json_string(json_array(1, 2, 0, -1, 1.1, -0.11)) | fields json_string
    fetched rows / total rows = 1/1
    +--------------------------------+
    | json_string                    |
    +--------------------------------+
    | [1.0,2.0,0.0,-1.0,1.1,-0.11]   |
    +--------------------------------+

    os> source=people | eval `json_string` = to_json_string(json_object('key', 123.45)) | fields json_string
    fetched rows / total rows = 1/1
    +-----------------+
    | json_string     |
    +-----------------+
    | {'key', 123.45} |
    +-----------------+

What alternatives have you considered?

N/A

Do you have any additional context?

opensearch-project/opensearch-spark#870 - PR to add json objects to opensearch-spark PPL