Enable better extensibility of this library

src/Grant/AuthCodeGrant.php

@@ -9,6 +9,7 @@
 
 namespace League\OAuth2\Server\Grant;
 
+use League\OAuth2\Server\Entities\AuthCodeEntityInterface;
 use League\OAuth2\Server\Entities\ClientEntityInterface;
 use League\OAuth2\Server\Entities\ScopeEntityInterface;
 use League\OAuth2\Server\Entities\UserEntityInterface;
@@ -172,6 +173,9 @@ public function respondToAccessTokenRequest(
             }
         }
 
+        // Handle extra authorization code parameters
+        $this->handleExtraAuthCodeParams($authCodePayload);
+
         // Issue and persist access + refresh tokens
         $accessToken = $this->issueAccessToken($accessTokenTTL, $client, $authCodePayload->user_id, $scopes);
         $refreshToken = $this->issueRefreshToken($accessToken);
@@ -338,6 +342,8 @@ public function completeAuthorizationRequest(AuthorizationRequest $authorization
                 'code_challenge_method' => $authorizationRequest->getCodeChallengeMethod(),
             ];
 
+            $payload = array_merge($this->getExtraAuthCodeParams($authorizationRequest, $authCode), $payload);
+
             $response = new RedirectResponse();
             $response->setRedirectUri(
                 $this->makeRedirectUri(
@@ -367,4 +373,27 @@ public function completeAuthorizationRequest(AuthorizationRequest $authorization
             )
         );
     }
+
+    /**
+     * Add custom fields to your authorization code to save some data from the previous (authorize) state
+     * for when you are issuing the token at the token endpoint
+     *
+     * @param AuthorizationRequest    $authorizationRequest
+     * @param AuthCodeEntityInterface $authCode
+     *
+     * @return array
+     */
+    protected function getExtraAuthCodeParams(AuthorizationRequest $authorizationRequest, AuthCodeEntityInterface $authCode)
+    {
+        return [];
+    }
+
+    /**
+     * Handle the extra params specified in getExtraAuthCodeParams
+     *
+     * @param object $authCodePayload
+     */
+    protected function handleExtraAuthCodeParams($authCodePayload)
+    {
+    }
 }

src/ResponseTypes/BearerTokenResponse.php

@@ -63,9 +63,8 @@ public function generateHttpResponse(ResponseInterface $response)
     }
 
     /**
-     * Add custom fields to your Bearer Token response here, then override
-     * AuthorizationServer::getResponseType() to pull in your version of
-     * this class rather than the default.
+     * Add custom fields to your Bearer Token response here, then pass an instance
+     * of your version of this class into the last parameter of the AuthorizationServer.
      *
      * @param AccessTokenEntityInterface $accessToken
      *