Querying shadow tables should return an error

[adsharma]

When we attach a foreign table, we create a shadow table for each of the tables in duckdb for the purpose of managing REL tables (which require unique table ids).

However, querying these shadow tables is possible today and the user could be confused with zero results.

Possibilities:

• Return a clear error
• Do not create a shadow table until a REL table is created. Even then, selectively create shadow tables.