[FEATURE] Add ARRAY_TO_CSV helper function

Is your feature request related to a problem?
Using PPL 3.4.0, it would be very convenient to have an array helper function to flatten an array content into a CSV string.

As of 2025-10-16, the PPL Collection Functions has a REDUCE function which can fulfill this task. However it is rather clumsy.

Example:

source=security-auditlog-2024.05
| eval myArray=array('GET', 'READ', 'WRITE')
| eval myArray_CSV = reduce(myArray, '', (acc, x) -> CONCAT_WS(',', acc, x))
| fields myArray, myArray_CSV
| head 1

The output looks like (notice the , prefix in the CSV result ,GET,READ,WRITE)

{
  "schema": [
    {
      "name": "myArray",
      "type": "array"
    },
    {
      "name": "myArray_CSV",
      "type": "string"
    }
  ],
  "datarows": [
    [
      [
        "GET",
        "READ",
        "WRITE"
      ],
      ",GET,READ,WRITE"
    ]
  ],
  "total": 1,
  "size": 1
}

What solution would you like?
It would be more convenient to have a utility function named ARRAY_TO_CSV to build the CSV string. The above query would have a more user-friendly syntax.

source=security-auditlog-2024.05
| eval myArray=array('GET', 'READ', 'WRITE')
| eval myArray_CSV = ARRAY_TO_CSV(myArray)
| fields myArray, myArray_CSV
| head 1

Furthermore, the ARRAY_TO_CSV should accept and optional 2nd parameter which is the delimiter string. This would allow a better formatting of the CSV string. The default value as , (comma, no space)

// CSV with comma-space as separator
ARRAY_TO_CSV(myArray, ", ")  -> "GET, READ, WRITE"

// CSV with space-pipe-space as separator
ARRAY_TO_CSV(myArray, " | ")  -> "GET | READ | WRITE"

What alternatives have you considered?
Use the REDUCE function, which has 2 inconveniences:

• complicate syntax
• resulting CSV always has an extra separator string, either at the beginning or the end.

Do you have any additional context?
The ARRAY_TO_CSV function if available, would help to improve the output of the collector operators TAKE, LIST, VALUES of the stats command.

Start with the collection functions documentation in docs/user/ppl/functions/collection.rst and compare the requested behavior with REDUCE, then review the related stats documentation in docs/user/ppl/cmd/stats.rst. Done means ARRAY_TO_CSV accepts an array, uses a comma by default, supports an optional delimiter, and produces the demonstrated strings without an extra separator.