DEV: Upload assets to S3 in parallel (#30210)

In my local setup (with Minio), this uploads the assets to S3 ~40% faster.

lib/tasks/s3.rake

@@ -194,7 +194,15 @@ task "s3:ensure_cors_rules" => :environment do
 end
 
 task "s3:upload_assets" => [:environment, "s3:ensure_cors_rules"] do
-  assets.each { |asset| upload(*asset) }
+  pool =
+    Concurrent::FixedThreadPool.new(
+      ENV["DISCOURSE_S3_UPLOAD_ASSETS_RAKE_THREAD_POOL_SIZE"] || Concurrent.processor_count,
+    )
+
+  assets.each { |asset| pool.post { upload(*asset) } }
+
+  pool.shutdown
+  pool.wait_for_termination
 end
 
 task "s3:expire_missing_assets" => :environment do