Snowflake: Add CREATE and DROP EXTERNAL VOLUME

[sabir-akhadov-localstack]

Snowflake's CREATE EXTERNAL VOLUME and DROP EXTERNAL VOLUME did not parse. Example that failed before:

CREATE EXTERNAL VOLUME my_vol STORAGE_LOCATIONS = (
    (NAME = 'loc1' STORAGE_PROVIDER = 'S3' STORAGE_BASE_URL = 's3://bucket/path/'));

This adds CREATE [OR REPLACE] EXTERNAL VOLUME [IF NOT EXISTS] and DROP EXTERNAL VOLUME (mirroring the merged CREATE/DROP WAREHOUSE, #2388). Each storage location is a parenthesized key-value option list parsed generically into the existing KeyValueOptions, so fields (including the nested ENCRYPTION = (...) list) may appear in any order, matching real Snowflake. Only an empty location is rejected; semantic checks such as the required STORAGE_BASE_URL or duplicate fields are left to the consumer, in line with the parser's permissive stance. The trailing ALLOW_WRITES and COMMENT properties are accepted in either order. DROP EXTERNAL VOLUME reuses the generic Statement::Drop via a new ObjectType::ExternalVolume. All statements round-trip.

See the Snowflake CREATE EXTERNAL VOLUME docs.

Replaces #2397, split into four PRs per review feedback — this one plus ALTER (#2474), DESC (#2472) and SHOW (#2473).

[sabir-akhadov-localstack]

Rebased on latest main. @iffyio @LucaCappelletti94 could you take a look when you have a moment? This is the first of the PRs split out of #2397 (#2472 and #2473 are the others; #2474 is stacked on this one).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.78082% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.23%. Comparing base (5d653a2) to head (fb55c05).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ast/mod.rs	77.77%	1 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2475      +/-   ##
==========================================
+ Coverage   81.16%   81.23%   +0.07%     
==========================================
  Files          42       42              
  Lines       33769    33944     +175     
  Branches    33769    33944     +175     
==========================================
+ Hits        27408    27575     +167     
+ Misses       2797     2791       -6     
- Partials     3564     3578      +14     

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