[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.

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

MailServiceProvider.php

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