Added extension methods to configure migrations.

Removed MigrationAssembly and MigrationNamespace from RelationalOptionsExtension.
Added a MigrationsOptionsExtensions instead.
Added MigrationsDbContextOptionsExtensions with the methods UseMigrationAssembly, UseMigrationNamespace.
Updated related code and unit tests, added a couple new unit tests.
I see that it is "hidden"  by the other extension methods. This then seems like an implementation detail that can be made private.