code: public static function getConfigDefinitions()
      {
          return [
              'apiVersion' => [
                  'valid' => ['string'],
                  'default' => \DTS\eBaySDK\OAuth\Services\OAuthService::API_VERSION,
                  'required' => true
              ],
              'profile' => [
                  'valid' => ['string'],
                  'fn'    => 'DTS\eBaySDK\applyProfile',
              ],
              'credentials' => [
                  'valid'   => ['DTS\eBaySDK\Credentials\CredentialsInterface', 'array', 'callable'],
                  'fn'      => 'DTS\eBaySDK\applyCredentials',
                  'default' => [CredentialsProvider::class, 'defaultProvider']
              ],
              'debug' => [
                  'valid'   => ['bool', 'array'],
                  'fn'      => 'DTS\eBaySDK\applyDebug',
                  'default' => false
              ],
              'httpHandler' => [
                  'valid'   => ['callable'],
                  'default' => 'DTS\eBaySDK\defaultHttpHandler'
              ],
              'httpOptions' => [
                  'valid'   => ['array'],
                  'default' => [
                      'http_errors' => false
                  ]
              ],
              'ruName' => [
                  'valid'    => ['string'],
                  'required' => true
              ],
              'sandbox' => [
                  'valid'   => ['bool'],
                  'default' => false
              ]
          ];
      }
docs: Returns definitions for each configuration option that is supported.
  
  @return array An associative array of configuration definitions.