FIX: Set sane default for Net::HTTP when processing a request (#28141)

This commit patches `Net::HTTP` to reduce the default timeouts of 60
seconds when we are processing a request. There are certain routes in
Discourse which makes external requests and if the proper timeouts are
not set, we risk having the Unicorn master process force restarting the
Unicorn workers once the `30` seconds timeout is reached. This can
potentially become a vector for DoS attacks and this commit is aimed at
reducing the risk here.

lib/middleware/processing_request.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class Middleware::ProcessingRequest
+  PROCESSING_REQUEST_THREAD_KEY = "discourse.processing_request"
+
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    Thread.current[PROCESSING_REQUEST_THREAD_KEY] = true
+    @app.call(env)
+  ensure
+    Thread.current[PROCESSING_REQUEST_THREAD_KEY] = nil
+  end
+end