fix: support getting table properties using schema with db name prefix

[guenp]

With the introduction of PR #835 a user might want to fully specify a schema with the database name as prefix, i.e. "my db"."my schema".

This PR fixes a bug that currently prevents usage of Dialect.has_table, Dialect.get_table_names, Dialect.get_columns, Dialect.get_foreign_keys and Dialect.get_check_constraints when using said fully specified schema name.

I also refactored test_get_schema_names so there is a separate test for get_table_names.

Edit: This fixes duckdb/duckdb#10033

[guenp]

@Mause, this is now ready for review. I added a method _pg_class_filter_scope_schema which filters the query by the specified schema. This only fixes the original bug partially, however - we don't cover the edge case where a schema + table with the same name exists in another database (see note on line 488). I suggest to address this in a subsequent PR.

[guenp]

Please review/merge this PR to address this issue: apache/superset#26286 duckdb/duckdb#10033