`pg_type` doesn't catalog user tables & `pg_class` doesn't store `reltype` column

Blocking Prisma. We currently support using user defined tables as composite types, but we cannot see them in `pg_type`. Ditto for dolt system tables, even with `dolt_show_system_tables` active.

Similarly we don't have any values set for the `reltype` column in pg_class, which would correspond with the associated row in `pg_type`.

I can look into this more tomorrow.

TODO comments [here](https://github.com/dolthub/doltgresql/blob/0873c7e5a4007210d0409126cb0d9a7ddb4d2b1e/core/typecollection/typecollection.go#L115) or [here](https://github.com/dolthub/doltgresql/blob/0873c7e5a4007210d0409126cb0d9a7ddb4d2b1e/core/typecollection/typecollection.go#L316) seem to be the places that would need to change.