support loading via staging for postgres with embedded duckdb (pg_duckdb)

[rudolfix]

Background We are implementing #1692 and the same code should work also for postgres with embedded duckdb: https://github.com/duckdb/pg_duckdb

We'll allow a fast loading of parquet/json/csv from buckets most probably without any additional code.

Requirements

1. • [ ] create tables using postgres table engine
2. • [ ] copy data into those tables with duckdb engine

support looks like early. I wonder if replication and backups work for duckdb tables