Snowflake: parse structured OBJECT types

[osipovartem]

Summary

• represent Snowflake structured OBJECT(field TYPE, ...) as a typed AST node
• parse nested OBJECT/ARRAY fields and NOT NULL constraints recursively
• preserve existing unstructured OBJECT behavior

Validation

• cargo +1.95.0 test --all-features
• cargo +1.95.0 clippy --all-targets --all-features -- -D warnings
• cargo +1.95.0 fmt --all -- --check