fix: don't introspect internal UNIQUE constraints

[IlyaFaer]

When a foreign key created, Spanner adds a UNIQUE constraint for the column under the hood. As the dialect introspection method returns this constraint, new autogenerated migration is trying to delete the constraint.

I think here we should do the same we did for indexes - if a constraint is ruled automatically by Spanner, we should not allow the dialect to see it. Still, information_schema doesn't include the direct attribute showing the constraint is ruled automatically. Thus, we have to do a JOIN to the table, where foreign keys are described, and see if this UNIQUE constraint is binded to a foreign key. If yes, we should hide it from the dialect to prevent Alembic from trying to delete it.

Fixes #231