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.
It cannot be made private because it is used in EntityFramework.Commands/MigrationTool.cs