sql ML command supports category

Description

From the document of ML command, it shows that ml supports category_field command, but actually it doesn't work. This PR makes ML command supports category_field parameter.

Request:

POST _plugins/_ppl?format=jdbc
{
  "query":"source = abcd_test | eval value = cast(value as double) | fields value, category | ml action='trainandpredict' algorithm='rcf' input='value' category_field='category'"
}

Response:

{
  "schema": [
    {
      "name": "value",
      "type": "double"
    },
    {
      "name": "category",
      "type": "string"
    },
    {
      "name": "score",
      "type": "double"
    },
    {
      "name": "anomalous",
      "type": "boolean"
    }
  ],
  "datarows": [
    [
      1,
      "a",
      0,
      false
    ],
    [
      2,
      "b",
      0,
      false
    ]
  ],
  "total": 2,
  "size": 2
}

Related Issues

https://github.com/opensearch-project/sql/issues/3406

Check List

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

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