Add read_tbl_duckdb()

[krlmlr]

Read a table from an arbitrary database file.

• read_tbl_duckdb() : should that function be able to read from an arbitrary DuckDB file? Alternative: read_db_duckdb()
• db_attach() and db_detach() (similarly to db_exec())
• compute(temporary = FALSE) checks if the target schema is provided and actually is attached to another database, errors otherwise
• Should compute() accept a file name? Alternative: compute_db()

[krlmlr]

Descoping.