Push down quals in foreign table scan

[rebasedming]

What
The begin_scan function of the foreign data wrapper ignores quals, which means that predicates are not pushed down to DuckDB in the case that the query falls back to the FDW scan.

Why
Pushing down quals will make queries that fall back to the FDW faster. Could be useful for federated joins.

How
Parse quals and add them to the DuckDB query. I believe the clickhouse_fdw has a good implementation for this.

[kysshsy]

I can handle this if no one else is already working on it. I implemented a simple code, and it seems to work.(I think it's not cover all types) Is the main challenge that DuckDB types are not entirely compatible with PostgreSQL?（and my code needs to cover all corner cases) (I am not familiar with duckdb :)

[rebasedming]

If you put up a draft PR with a description of the issue I can provide more pointed feedback.

[philippemnoel]

I can handle this if no one else is already working on it. I implemented a simple code, and it seems to work.(I think it's not cover all types) Is the main challenge that DuckDB types are not entirely compatible with PostgreSQL?（and my code needs to cover all corner cases) (I am not familiar with duckdb :)

No one else is working on it. We'd love for you to handle this feature :)

[philippemnoel]

Solved by #39, thank you @kysshsy!