Add Cache-Control header to openstack-stored files (#20610)

When storing files in S3, paperclip is configured with a Cache-Control header
indicating the file is immutable, however no such header was added when using
OpenStack storage.

Luckily Paperclip's fog integration makes this trivial, with a simple
`fog_file` `Cache-Control` default doing the trick.

config/initializers/paperclip.rb

@@ -126,6 +126,8 @@ elsif ENV['SWIFT_ENABLED'] == 'true'
       openstack_cache_ttl: ENV.fetch('SWIFT_CACHE_TTL') { 60 },
     },
     
+    fog_file: { 'Cache-Control' => 'public, max-age=315576000, immutable' },
+
     fog_directory: ENV['SWIFT_CONTAINER'],
     fog_host: ENV['SWIFT_OBJECT_URL'],
     fog_public: true