MockHttpClient::getConfig must be compatible with GuzzleHttp\ClientInterface::getConfig

Problem/Motivation

We are getting an error running kernel tests with video_embed_field enabled:

PHP Fatal error:  Declaration of Drupal\Tests\video_embed_field\Kernel\MockHttpClient::getConfig(?string $option = null) must be compatible with GuzzleHttp\ClientInterface::getConfig($option = null) in /modules/contrib/video_embed_field/tests/src/Kernel/MockHttpClient.php on line 51

Proposed resolution

Change MockHttpClient::getConfig to respect ClientInterface::getConfig