Allow overriding parts of a client config

[prudloff-insite]

Assuming two different config files:

knpu_oauth2_client:
    clients:
        drupal:
            client_id: foo
            client_secret: foo
            type: drupal
            redirect_route: foo
            base_url: foo

imports:
    - { resource: config.yml }

knpu_oauth2_client:
    clients:
         drupal:
             client_id: bar

This will cause an InvalidConfigurationException:

Unrecognized option "0" under "knpu_oauth2_client/clients/drupal"

This PR fixes this error and allows overriding parts of the configuration.

(See symfony/symfony#12304 (comment) for a more detailed explanation.)

[weaverryan]

Thank you @prudloff-insite!

[weaverryan]

Tagged 1.29.2 :)