PPL: Add json_object command

[acarbonetto]

This PR depends on the json_valid command: #3230

Description

For OpenSearch-PPL, adds the json_object() functions that takes a even number of varargs (key/value pairs). Returns an expression type that is valid json object (STRUCT).

Description
>>>>>>>>>>>

Usage: `json_object(<key>, <value>[, <key>, <value>]...)` returns a JSON object from key-value pairs.

Argument type:
- A \<key\> must be STRING.
- A \<value\> can be a scalar, another json object, or json array type.  Note: scalar fields will be treated as single-value.  Use `json_array` to construct an array value from a multi-value.

Return type: STRUCT

Example:

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

    os> source=people | eval result = json_object('outer', json_object('inner', 123.45)) | fields result
    fetched rows / total rows = 1/1
    +------------------------------+
    | result                       |
    +------------------------------+
    | {"outer":{"inner":123.45}}   |
    +------------------------------+

    os> source=people | eval result = json_object('array_doc', json_array(123.45, "string", true, null)) | fields result
    fetched rows / total rows = 1/1
    +------------------------------------------------+
    | result                                         |
    +------------------------------------------------+
    | {"array_doc":[123.45, "string", true, null]}   |
    +------------------------------------------------+

Related Issues

Resolves #3208

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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