Allow models to be backed by foreign tables in PostgreSQL for undefined_table_references

[fatkodima]

We can avoid creating our own method to get the VIEWs, since data_source_exists? already does this. And additionally this will allow for models to be backed by foreign tables in PostgreSQL.

https://github.com/rails/rails/blob/be400c3ca2b86111766d840979478fc026679368/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb#L882-L909