MySQL: Implement (key_part, ...) in index definitions

[yoichi]

It also adds support for index prefixes:
https://dev.mysql.com/doc/refman/8.0/en/column-indexes.html#column-indexes-prefix

Brief summary of the change made

fixes #3886

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

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.

[greg-finley]

Thanks!

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (547f116) compared to base (c94e47c).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #3887   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          184       184           
  Lines        14147     14147           
=========================================
  Hits         14147     14147           

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

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