Add support for *unenforced* primary and foreign keys

[gahtan-syarif]

Bigquery, databricks, and snowflake has support for unenforced primary and foreign keys. While it is not enforced, it is also not purely for decoration, as it opens the door for query engines to better optimize for JOIN queries as seen here. On top of that, this would allow ducklake to better integrate with ERD creation tools. This feature should be trivial to implement as duckdb does not have to enforce the constraints.