AuthenticationManager::getSortedProviders is protected

As pointed out by @alexpott in #2456303-16: AuthenticationManager needs interface and ::getProviderKeys a better way to collect the keys is to just collect them from the container using a service collector. Note that we currently are working on #2432585: Improve authentication manager service construction to support custom global service providers, please chime in over there.

1. Implement two new methods in an existing service (or add a new one):
   • addProvider($id)
   • getProviderKeys()
2. Make sure to add the service_collector tag to the service with the new addProvider() method
3. Use the getProviderKeys() from your own service wherever you wish.

Example of the service definition:

services:
  restui.providercollector:
    class: Drupal\restui\RestUIProviderCollector
    tags:
    - { name: service_collector, tag: authentication_provider, call: addProvider }

Example of the class:

class RestUIProviderCollector {
  protected $keys;
  public function addProvider($provider_id) {
    $keys[] = $provider_id;
  }
  public function getProviderKeys() {
    return $keys;
  }
}

Note that we are planning to introduce explicit provider keys instead of the magic applied to the service id in #2432585. This will render the convertServiceIdToProviderKey method superflous. Update: #2432585: Improve authentication manager service construction to support custom global service providers landed, provider_id argument now can be used.

Slightly renamed the collector and copied a few bits from the Authentication Manager over to make sure we get the same behavior.

Here's a removal of custom collector in favour of core one that here since 8.0

PS: + additional code clean-up

@clemens.tolboom Filed #2744445: Clean-up codebase according current code standards