Snowflake - ALTER TABLE IF EXISTS and WHEN SYSTEM$STREAM_HAS_DATA()

[chrisalexeev]

Brief summary of the change made

Added snowflake dialect support for the optional ALTER TABLE IF EXISTS... syntax as well as tasks that use the WHEN SYSTEM$STREAM_HAS_DATA() conditional.

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

No.

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 #3641 (39eb2a5) into main (ac852a2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3641   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          175       175           
  Lines        13294     13297    +3     
=========================================
+ Hits         13294     13297    +3     

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

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