Snowflake: Support ALTER TABLE ADD COLUMN IF NOT EXISTS

[sabir-akhadov-localstack]

Snowflake supports IF NOT EXISTS on ALTER TABLE ... ADD [COLUMN] (docs), but this failed to parse with SnowflakeDialect:

ALTER TABLE tab ADD COLUMN IF NOT EXISTS c TEXT

The check was a hardcoded dialect_of!(PostgreSql | BigQuery | DuckDb | Generic). This replaces it with a new Dialect::supports_add_column_if_not_exists() method, enables it for those dialects plus Snowflake, and switches the existing common test to all_dialects_where.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.20%. Comparing base (5d653a2) to head (4087bc8).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2639      +/-   ##
==========================================
+ Coverage   81.16%   81.20%   +0.04%     
==========================================
  Files          42       42              
  Lines       33769    33890     +121     
  Branches    33769    33890     +121     
==========================================
+ Hits        27408    27520     +112     
+ Misses       2797     2796       -1     
- Partials     3564     3574      +10     

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