Fix caching headers in ActivityPub endpoints (#11331)

* Fix reverse-proxy caching in public fetch mode

* Fix caching in ActivityPub-specific controllers

app/controllers/activitypub/base_controller.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class ActivityPub::BaseController < Api::BaseController
+  private
+
+  def set_cache_headers
+    response.headers['Vary'] = 'Signature' if authorized_fetch_mode?
+  end
+end