[FEATURE] Ad json_keys function to PPL

[acarbonetto]

Is your feature request related to a problem?

As part of the RFC to add JSON functions, the json_keys function would be useful to extract a list of keys at a path from a json documents

• 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?

### `JSON_KEYS`

**Description**

`json_keys(doc [, path])` Returns an array of all keys of the object at path in the JSON document.  If path is not specified, returns the keys at the root of the JSON document. 

**Argument type:**  OBJECT, STRING

A STRING expression of a valid JSON document.

**Return type:** ARRAY

`json_keys(doc, paths)` Returns an array of all keys of the object at the given paths in the JSON document.  

**Argument type:**  OBJECT, ARRAY

A STRING expression of a valid JSON document.

**Return type:** ARRAY

`NULL` is returned from an invalid JSON.

Example:

    os> source=people | eval `keys` = json_keys(json('{"f1":"abc","f2":{"f3":"a","f4":"b"}}'))
    fetched rows / total rows = 1/1
    +------------+
    | keys       |
    +------------+
    | [f1, f2]   |
    +------------+

    os> source=people | eval `keys` = json_keys(json('{"f1":"abc","f2":{"f3":"a","f4":"b"}}'), "f1", "f2")
    fetched rows / total rows = 1/1
    +------------+
    | keys       |
    +------------+
    | [f3, f4]   |
    +------------+

    os> source=people | eval `keys` = json_keys(json('{"f1":"abc","f2":{"f3":"a","f4":"b"}}'), json_array("{}", "f2"))
    fetched rows / total rows = 1/1
    +------------+
    | keys       |
    +------------+
    | [f1, f2, f3, f4]   |
    +------------+

    os> source=people | eval `keys` = json_keys(json('[1,2,3,{"f1":1,"f2":[5,6]},4]'))
    fetched rows / total rows = 1/1
    +--------+
    | keys   |
    +--------+
    | null   |
    +--------+

What alternatives have you considered?

N/A

Do you have any additional context?

opensearch-project/opensearch-spark#780 - PR to add json objects to opensearch-spark PPL