Issue with guzzle configuration

[kidager]

Issue / Motivation:

When additionalConfigKeys is defined in a provider, the guzzle configuration is dropped.

Expected behavior:

When calling ConfigRetriever::fromServices, ConfigRetriever->additionalConfigKeys should contain the guzzle key if it was defined in the service.

Actual behavior:

When additionalConfigKeys is defined in a provider, the guzzle configuration is dropped.

Steps to reproduce the behavior:

In this example, the guzzle is defined in the service configuration, however, since the provider defines the additionalConfigKeys method, guzzle configuration gets ignored.

• SampleProvider.php

  // ...
  class SampleProvider extends AbstractProvider
  {
      // ...
      public static function additionalConfigKeys()
      {
          return ['config1', 'config2'];
      }
      // ...
  }

• Listener

  $socialiteWasCalled->extendSocialite('sample-provider', SampleProvider::class);

• config/services.php

  'sample-provider' => [
      'client_id' => 'sample-provider-client-id',
      'client_secret' => 'sample-provider-client-secret',
      'redirect' => 'sample-provider-redirect-url',
      'config1' => 'foo',
      'realms' => 'bar',
      'guzzle' => [
          'verify' => false,
      ],
  ],

Proposed solution:

See #227