Comma-separated index matching

[eugenesk24]

Is your feature request related to a problem? Please describe.
Users need the ability to query multiple indices with different names.

Describe the solution you'd like
Support for comma-separated index matching

Describe alternatives you've considered
We currently have the ability to use the wildcard pattern but this limits users to only include indices with similar naming schemas.

Additional context
Currently this works,

PUT /name/_doc/1
{
  "field": "123"
}

PUT /name_different/_doc/1
{
  "different_field": "abc"
}

POST /_plugins/_ppl
{
  "query" : """
          source = name* | fields different_field
        """
}

which gives,

{
  "schema": [
    {
      "name": "different_field",
      "type": "string"
    }
  ],
  "datarows": [
    [
      null
    ],
    [
      "abc"
    ]
  ],
  "total": 2,
  "size": 2
}

But this does not,

PUT /name/_doc/1
{
  "totally_field": "123"
}

PUT /name_different/_doc/1
{
  "different_field": "abc"
}

POST /_plugins/_ppl
{
  "query" : """
          source = name, name_different | fields different_field
        """
}

which gives,

{
  "error": {
    "reason": "Invalid Query",
    "details": "Failed to parse query due to offending symbol [,] at: '\n          source = name,' <--- HERE... More details: Expecting tokens in {<EOF>, '|', '.'}",
    "type": "SyntaxCheckException"
  },
  "status": 400
}