FEATURE: Allow oneboxing private GitHub URLs (#27705)

This commit adds the ability to onebox private GitHub
commits, pull requests, issues, blobs, and actions using
a new `github_onebox_access_token` site setting. The token
must be set up in correctly to have access to the repos needed.

To do this successfully with the Oneboxer, we need to skip
redirects on the github.com host, otherwise we get a 404
on the URL before it is translated into a GitHub API URL
and has the appropriate headers added.

lib/onebox/engine/github_pull_request_onebox.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative "../mixins/github_body"
+require_relative "../mixins/github_auth_header"
 
 module Onebox
   module Engine
@@ -9,6 +10,7 @@ module Onebox
       include LayoutSupport
       include JSON
       include Onebox::Mixins::GithubBody
+      include Onebox::Mixins::GithubAuthHeader
 
       GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/
 
@@ -27,7 +29,7 @@ module Onebox
       end
 
       def data
-        result = raw.clone
+        result = raw(github_auth_header).clone
         result["link"] = link
 
         created_at = Time.parse(result["created_at"])