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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[YANG-DB]

@acarbonetto a naive question - do we have more use cases of kv pairs such as this one ?
if so would it make sense to move this code into a more generic place?

[acarbonetto]

Interesting question - no. json_object is very unique. Although we do have cases of path-value pairings, we're going to treat them differently.

[YANG-DB]

can you plz add more complex nested use cases here ?
for example inner obj with and array side by an array ?

[acarbonetto]

We need the json_array function to do that. I've added a TODO in the tests to add this test in the future.

[Yury-Fridlyand]

What is the purpose of that API? It looks kinda useless.
Are there future plans for other features where it could be used?

[acarbonetto]

What is the purpose of that API? It looks kinda useless. Are there future plans for other features where it could be used?

As mentioned in the RFC, this API mirrors the Splunk SP2 api and would be helpful for migrating to sharing data between apis.