Snowflake: Add SHOW EXTERNAL VOLUMES

[sabir-akhadov-localstack]

Snowflake's SHOW EXTERNAL VOLUMES did not parse:

SHOW EXTERNAL VOLUMES LIKE 'my_%';

This adds a ShowExternalVolumes statement to the Snowflake dialect, reusing the existing ShowStatementFilter for the optional LIKE clause. All statements round-trip.

See the Snowflake SHOW EXTERNAL VOLUMES docs.

Split out of #2397 per review feedback, alongside the CREATE/DROP (#2475), ALTER (#2474), and DESC (#2472) PRs.

[sabir-akhadov-localstack]

Rebased on latest main. @iffyio @LucaCappelletti94 could you take a look when you have a moment? (One of the PRs split out of #2397.)

[LucaCappelletti94]

At this time we are rather overworked review wise, so I'll get to it when I'll get to it. I would appreciate if you could help out with reviewing other PRs, as it would accelerate our work and ultimately speed up the PR queue.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.20%. Comparing base (5d653a2) to head (070d23f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ast/mod.rs	81.81%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2473      +/-   ##
==========================================
+ Coverage   81.16%   81.20%   +0.03%     
==========================================
  Files          42       42              
  Lines       33769    33891     +122     
  Branches    33769    33891     +122     
==========================================
+ Hits        27408    27520     +112     
+ Misses       2797     2796       -1     
- Partials     3564     3575      +11     

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