I'm a plumber and I'm OK...

Plumb new sequences API through stack

This includes connecting up to Migrations and value generation and the removal of old APIs, plus some changes to the original implementation driven by usage patterns for these things.
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.