[LucaCappelletti94]

ALTER EXTENSION citext SET SCHEMA other, and every other form of the statement, fails with Expected: one of VIEW or MATERIALIZED VIEW or ..., found: EXTENSION, because parse_alter has no EXTENSION arm.

This adds Statement::AlterExtension, which holds the extension name and an AlterExtensionOperation whose first variant is SetSchema, shaped like the one of ALTER COLLATION.

It is the first PR of what I expect to be a rather high stack of PRs that adds the ALTER EXTENSION grammar.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.30%. Comparing base (96a5132) to head (b1a99a8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2913      +/-   ##
==========================================
+ Coverage   83.28%   83.30%   +0.02%     
==========================================
  Files          42       42              
  Lines       35355    35384      +29     
  Branches    35355    35384      +29     
==========================================
+ Hits        29446    29478      +32     
+ Misses       2458     2455       -3     
  Partials     3451     3451              

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