[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.
If you're going down the contract route then your public API should be a part of the contract. All of it. Otherwise one implementation will be different to another. What's the point of a contract if that's going to be the case?