Allow marking tables as existing

[mwstr]

There is a mechanism for pg roles that allows marking them as existing

pgRole('admin').existing();

which excludes them from migrations.

It would be great to have the same mechanism for tables. A common use case for this feature is Supabase tables like auth.users which are already existing in database.

For now I'm adding this tables to schema and editing generated migration files, so they don't create such tables. But this approach is error-prone in many ways