Diagnose command: Add GitHub OAuth token expiration date information

GitHub's new fine-grained tokens have a cumpulsory expiration date, and their
classic tokens also support an expiration date.

https://github.blog/changelog/2021-07-26-expiration-options-for-personal-access-tokens/

This improves the `composer diagnose` command to display the expiration
date and time if it is provided by the response headers
(via `GitHub-Authentication-Token-Expiration`).

src/Composer/Command/DiagnoseCommand.php

@@ -327,10 +327,20 @@ private function checkGithubOauth(string $domain, string $token)
         try {
             $url = $domain === 'github.com' ? 'https://api.'.$domain.'/' : 'https://'.$domain.'/api/v3/';
 
-            $this->httpDownloader->get($url, [
+            $response = $this->httpDownloader->get($url, [
                 'retry-auth-failure' => false,
             ]);
 
+            $expiration = $response->getHeader('github-authentication-token-expiration');
+
+            if ($expiration === null) {
+                return '<info>OK</> <comment>no expiration</>';
+            }
+
+            if (\DateTime::createFromFormat('Y-m-d h:i:s O', $expiration)) {
+                return '<info>OK</> <comment>expires on '. $expiration .'</>';
+            }
+
             return true;
         } catch (\Exception $e) {
             if ($e instanceof TransportException && $e->getCode() === 401) {