Support Snowflake ALTER STAGE syntax - #34

Summary

• add Snowflake ALTER STAGE ... RENAME TO ... AST and parsing
• add structured ALTER STAGE ... SET support for stage, directory, file-format, copy, and comment properties
• share stage-option parsing with CREATE STAGE and reject empty/unsupported operations instead of accepting a no-op

ALTER STAGE ... REFRESH is intentionally not included because it requires a real directory metadata refresh implementation in the execution layer.

Validation

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