PERF: Improve cook_url performance for topic thumbnails (#11609)

- Only initialize the S3Helper when needed
- Skip initializing the S3Helper for S3Store#cdn_url
- Allow cook_url to be passed a `local` hint to skip unnecessary checks

spec/components/file_store/s3_store_spec.rb

@@ -6,7 +6,7 @@ require 'file_store/local_store'
 
 describe FileStore::S3Store do
   let(:store) { FileStore::S3Store.new }
-  let(:s3_helper) { store.instance_variable_get(:@s3_helper) }
+  let(:s3_helper) { store.s3_helper }
   let(:client) { Aws::S3::Client.new(stub_responses: true) }
   let(:resource) { Aws::S3::Resource.new(client: client) }
   let(:s3_bucket) { resource.bucket("s3-upload-bucket") }