Snowflake: Add DESC[RIBE] EXTERNAL VOLUME

[sabir-akhadov-localstack]

Snowflake's DESC[RIBE] EXTERNAL VOLUME did not parse:

DESCRIBE EXTERNAL VOLUME my_vol;

This adds a DescribeExternalVolume statement to the Snowflake dialect. The DESC vs DESCRIBE spelling is preserved on round-trip via the existing DescribeAlias.

See the Snowflake DESC EXTERNAL VOLUME docs.

Split out of #2397 per review feedback, alongside the CREATE/DROP (#2475), ALTER (#2474), and SHOW (#2473) 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.)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.20%. Comparing base (5d653a2) to head (4710f39).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2472      +/-   ##
==========================================
+ Coverage   81.16%   81.20%   +0.04%     
==========================================
  Files          42       42              
  Lines       33769    33900     +131     
  Branches    33769    33900     +131     
==========================================
+ Hits        27408    27530     +122     
+ Misses       2797     2796       -1     
- Partials     3564     3574      +10     

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