Make duckdb_fdw a sub-extension of pg_duckdb

[Vonng]

There used to be three extensions competing for the same libduckdb.
pg_mooncake has now been transitioned into a sub-extension of pg_duckdb, which resolves part of the conflict. The remaining overlap is with duckdb_fdw.

Would you consider either:

making duckdb_fdw a sub-extension that links against pg_duckdb’s bundled libduckdb, so both extensions can be installed simultaneously; This approach would remove the remaining collision while keeping both extensions installable side-by-side.