feat: propagate Postgres table and column comments through DuckDB metadata

[anirudhp-ops]

Summary

Closes #450

This PR adds support for Postgres schema comments to show up when
querying DuckDB metadata functions.

Changes

Modified GetInitializeQuery in postgres_table_set.cpp to join
against pg_description system catalog, surfacing both table-level
and column-level comments.

• Table comments are now visible via duckdb_tables()
• Column comments are now visible via duckdb_columns()

Testing
Verified manually by setting comments in Postgres and querying
DuckDB metadata:

sql
-- Set comments in Postgres
COMMENT ON TABLE test_comments IS 'this is a test table comment';
COMMENT ON COLUMN test_comments.name IS 'this is a column comment';

-- Verify in DuckDB
SELECT comment FROM duckdb_tables() WHERE table_name = 'test_comments';
-- Returns: 'this is a test table comment'

SELECT column_name, comment FROM duckdb_columns() WHERE table_name = 'test_comments';
-- Returns: name | 'this is a column comment'