Add Flint drop index SQL support

[dai-chen]

Description

Add Flint SQL support by ANTLR. Use DROP SKIPPING INDEX as first statement for simplicity.

Note that:

1. Separate FlintSpark.g4 (Flint extended SQL syntax) and SparkSqlBase.g4 (Spark identifier rule and lexical tokens)
2. Follow Spark command convention and returns empty from Flint Create/Drop DDL too.

Issues Resolved

opensearch-project/opensearch-spark#2

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.

[codecov]

Codecov Report

Merging #1730 (32d7320) into feature/flint (e80cf9b) will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             feature/flint    opensearch-project/sql#1730   +/-   ##
================================================
  Coverage            97.19%   97.19%           
  Complexity            4107     4107           
================================================
  Files                  371      371           
  Lines                10464    10464           
  Branches               706      706           
================================================
  Hits                 10170    10170           
  Misses                 287      287           
  Partials                 7        7           

Flag	Coverage Δ
sql-engine	97.19% <ø> (ø)

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