[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.
Personally I think `bindShared` should be in the contract (even though it was rejected in a recent PR), and Laravel should use `bindShared` in its providers.

Then this should type hint the container contract.