[SMALL] RelationalDatabase.ApplyMigrations

Adding an ApplyMigrations extension method to RelationalDatabase
Adding an interface that can be used to expose ContextConfiguration as an explicit implementation (useful for other components we want to hand extension methods off)
There is some level of testing as I swapped the MigrationsEndPointMiddleware over to use this API (and it has functional tests). Would be good to have at least one end-to-end test once we have migrations functional tests.
There may be objections to this name. :-)