[5.0] Add A Service Provider Contract

Allows service providers that do not extend `Support\ServiceProvider` but instead implement the contract, in the spirit of the new Contracts package.

- new ServiceProvider contract
- Foundation\Application typehints on the new contract
- call `$provider->register()' is now resolved using `->call()` so that ServiceProviders not extending `Support\ServiceProvider` can still use dependency injection.
ok, I understand that. But that doesn't mean you ditch typehinting, it means you write a wrapper for your Symfony DI class which implements the `Container` contract, or whatever. This way, stuff that's using that code knows what it's dealing with.