[feat req]: Merging MotherDuck databases other than the default db into postgres public schema

[naoyak]

When using pg_duckdb with MotherDuck, we'd like to select a db other than the internally configured default (my_db) to merge into the postgres.public schema.

Current behavior:

1. Each schema in your default MotherDuck database are simply merged with the Postgres schemas with the same name.
2. Except for the `main` DuckDB schema in your default database, which is merged with the Postgres `public` schema.
3. Tables in other databases are put into dedicated DuckDB-only schemas. These schemas are of the form `ddb$<duckdb_db_name>$<duckdb_schema_name>` (including the literal `$` characters).
4. Except for the `main` schema in those other databases. That schema should be accessed using the shorter name `ddb$<db_name>` instead.

Since there isn't a user-facing public API for a MotherDuck user to reassign the "default" database, we're stuck with the verbose ddb$other_md_db$schema naming scheme if we want to consume other schemas/tables downstream. Could we add a configuration setting e.g. in postgresql.conf to manually select which MotherDuck db to use as the "default"?

[JelteF]

Thanks for the report, we realized this was a problem as well and are planning to add an option for this soon to the postgresql.conf file.

[naoyak]

Thanks @JelteF, opened #470.

[naoyak]

Added in #470