Spark3: Support for EXPLAIN statement

[R7L208]

Brief summary of the change made

Replace ExplainStatementSegment from ANSI to allow for additional parameters and an expanded list of explainable statements.

Are there any other side effects of this change that we should be aware of?

Removed parse_grammar from ExplainStatementSegment in ANSI and re-assigned parse_grammar to match_grammar in Postgres and Snowflake dialects.

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #2767 (d3a0d35) into main (fa249a9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #2767   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          163       163           
  Lines        12206     12209    +3     
=========================================
+ Hits         12206     12209    +3     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_ansi.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_postgres.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_snowflake.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_spark3.py	100.00% <100.00%> (ø)

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[tunetheweb]

A few comments

[R7L208]

Updated PR description to reflect most recent change of removing parse_grammar from ExplainStatementSegment

[tunetheweb]

Thanks for adding those changes!