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_blob_onebox.rb

@@ -1,10 +1,13 @@
 # frozen_string_literal: true
 
 require_relative "../mixins/git_blob_onebox"
+require_relative "../mixins/github_auth_header"
 
 module Onebox
   module Engine
     class GithubBlobOnebox
+      include Onebox::Mixins::GithubAuthHeader
+
       def self.git_regexp
         %r{^https?://(www\.)?github\.com.*/blob/}
       end
@@ -35,6 +38,10 @@ module Onebox
       def title
         Sanitize.fragment(Onebox::Helpers.uri_unencode(link).sub(%r{^https?\://github\.com/}, ""))
       end
+
+      def auth_headers
+        github_auth_header
+      end
     end
   end
 end