Support Snowflake stage metadata statements

[osipovartem]

Summary

• parse Snowflake SHOW STAGES with LIKE and IN options
• parse DESC[RIBE] STAGE <name> as a dedicated AST statement
• gate both statements behind Snowflake dialect capabilities

Validation

• cargo test --all-features --tests
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --check
• git diff --check

Snowflake references:

• https://docs.snowflake.com/en/sql-reference/sql/show-stages
• https://docs.snowflake.com/en/sql-reference/sql/desc-stage