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.
This can't return an IDbContextOptions because other calls chained after it will see IDbContextOptions rather than DbContextOptions, as we discussed.