Change return type of the `CloneWith` method

[TechnoBerry]

Currently, the CloneWith method of the INpgsqlRelationalConnection interface has a concrete NpgsqlRelationalConnection class as the return type. It would be good to replace the return type with INpgsqlRelationalConnection.

[roji]

@TechnoBerry INpgsqlRelationalConnection is an internal type, and this method is for the provider's internal purposes - why do you think it should be changed?

[TechnoBerry]

@roji hello! In my work, I had a need to use an alternative implementation of this interface. This is because we use an internal DbConnection creation factory, which returns a wrapper over the NpgsqlConnection that we should use. Since we use pure sql for database migration, we do not need to override the CloneWith and CreateAdminConnection methods. It just seemed strange to me that the CreateAdminConnection method returns an interface, and the CloneWith method returns a specific implementation.

[roji]

OK thanks, submitted #3144 to clean this up.