Postgres: Add ALTER TYPE support

[greg-finley]

Brief summary of the change made

Adds the most common things for ALTER TYPE. It's probably enough to close the issue. I can add the remaining things when I get a chance.

Closes #3216

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

I put it under dialect_ansi to match where CreateTypeStatementSegment is. It's also fine if we want to move it to dialect_postgres.

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.

[tunetheweb]

I wonder should be moved to the Postgres dialect.

I've had a look around and very few dialects support custom types (so think we should move the CreateTypeSegment to Postgres too btw, and only Postgres seems to support ALTER TYPE.

Snowflake also removes the CreateTypeStatementSegment so similarly we should remove AlterTypeStatementSegment if we're not moving this to Postgres, but think we should move it.

WDYT?

[codecov]

Codecov Report

Merging #3265 (5642620) into main (03d9799) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3265   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          167       167           
  Lines        12539     12542    +3     
=========================================
+ Hits         12539     12542    +3     

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

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[greg-finley]

Yeah, I'll move both 👍