[ES|QL] add validation_only option

[drewdaemon]

Adds a validation_only option to the request body for _query.

If validation_only is true, the execution ends after the pre-analysis and analysis stages and returns

• validation errors if there are any
• an empty table if there aren't

curl -H 'Content-Type: application/json' \
      -d '{ "query": "FROM logs-aws_s3 |  EVAL aws.s3.bucket.name", "validation_only": true }' \
      -X POST \
      'http://localhost:9200/_query?format=txt'