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

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

app/controllers/activitypub/collections_controller.rb

 # frozen_string_literal: true
 
-class ActivityPub::CollectionsController < Api::BaseController
+class ActivityPub::CollectionsController < ActivityPub::BaseController
   include SignatureVerification
   include AccountOwnedConcern
 

app/controllers/activitypub/outboxes_controller.rb

 # frozen_string_literal: true
 
-class ActivityPub::OutboxesController < Api::BaseController
+class ActivityPub::OutboxesController < ActivityPub::BaseController
   LIMIT = 20
 
   include SignatureVerification

app/controllers/activitypub/replies_controller.rb

 # frozen_string_literal: true
 
-class ActivityPub::RepliesController < Api::BaseController
+class ActivityPub::RepliesController < ActivityPub::BaseController
   include SignatureAuthentication
   include Authorization
   include AccountOwnedConcern

app/controllers/application_controller.rb

@@ -160,6 +160,6 @@ class ApplicationController < ActionController::Base
   end
 
   def set_cache_headers
-    response.headers['Vary'] = 'Accept, Signature'
+    response.headers['Vary'] = public_fetch_mode? ? 'Accept' : 'Accept, Signature'
   end
 end