Provide a validated credential configuration

Problem/Motivation

The method of providing a PHP array directly for credentials when creating a SearchServiceClient is deprecated:

@type FetchAuthTokenInterface|CredentialsWrapper $credentials
     *           This option should only be used with a pre-constructed
     *           {@see FetchAuthTokenInterface} or {@see CredentialsWrapper} object. Note that
     *           when one of these objects are provided, any settings in $credentialsConfig will
     *           be ignored.
     *           **Important**: If you are providing a path to a credentials file, or a decoded
     *           credentials file as a PHP array, this usage is now DEPRECATED. Providing an
     *           unvalidated credential configuration to Google APIs can compromise the security
     *           of your systems and data. It is recommended to create the credentials explicitly
     *           ```
     *           use Google\Auth\Credentials\ServiceAccountCredentials;
     *           use Google\Cloud\DiscoveryEngine\V1\SearchServiceClient;
     *           $creds = new ServiceAccountCredentials($scopes, $json);
     *           $options = new SearchServiceClient(['credentials' => $creds]);

Proposed resolution

Update to the new recommended method to provide credentials.