Allow metadata fields in PPL query

[LantaoJin]

Description

OpenSearch reserved fields (_id, _index, _sort, _score, _max_score) are not allowed to be used in PPL clauses because the field format starting with underscore _ is not allowed.

• This ticket adds specific identifiers to the language, and opens up support for OpenSearch reserved identifiers.
• As an aside, identifiers with double underscore at the start (such as __myCoolField) is acceptable as an identifier.

#1456 had fixed the same problem for SQL query, but the issue still existed in PPL.

Issues Resolved

Resolves #2788

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[dai-chen]

Thanks for the fix!

[LantaoJin]

Gently ping @penghuo @vamsi-amazon @YANG-DB