feat: register webhooks route macro in ServiceProvider::register()

[misaf]

Problem

When Route::macro('webhooks', …) was declared in boot(), any other package that loads its routes in its own boot() could execute before WebhookClientServiceProvider finished booting.

In that situation a route file such as

// routes/api.php inside OtherPackage
Route::webhooks('stripe');

blew up with “Call to undefined method Illuminate\Routing\Router::webhooks”.

[freekmurze]

Thanks!