[5.8] Make register method in the ServiceProvider as part of the co…

…ntract

 In the laravel/framework#27067 pr was added DeferrableProvider service provider.
 This Pr will add pretty the same thing.
 In case, if it will be accepted then I will add documentation for this.

----------------

Also i thing will be good to add diferent contract for the:
- method `boot`
- property `singletons` and `bindings`

HashServiceProvider.php

@@ -4,8 +4,9 @@
 
 use Illuminate\Support\ServiceProvider;
 use Illuminate\Contracts\Support\DeferrableProvider;
+use Illuminate\Contracts\Support\RegistrableProvider;
 
-class HashServiceProvider extends ServiceProvider implements DeferrableProvider
+class HashServiceProvider extends ServiceProvider implements DeferrableProvider, RegistrableProvider
 {
     /**
      * Register the service provider.