Snowflake: parse nested COPY transformation paths

[osipovartem]

Summary

Allow Snowflake COPY INTO ... FROM (SELECT ...) transformations to parse staged fields followed by nested semi-structured paths or casts, for example:

SELECT $1:continent::VARCHAR, $1:country:name::VARCHAR
FROM @sf_tut_stage/cities.parquet

The dedicated StageLoadSelectItem parser remains the fast path for simple $1:key items. When more path/cast tokens follow, speculative parsing now rolls back to the standard expression parser so the complete JsonAccess / Cast AST is preserved.

Validation

• cargo test --test sqlparser_snowflake: 169/169
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

The regression query is based on Snowflake's documented Parquet COPY transformation syntax:
https://docs.snowflake.com/en/user-guide/tutorials/script-data-load-transform-parquet