Add CI remote include parallelization

What does this MR do and why?

Related to #351250 (closed)

In preload_content, we start executing the HTTP call in another thread. Then, when the content method is called, we wait for the HTTP response.

This is the first step. In the next step, we'll work on limiting the max number of parallel HTTP calls in a single-user call.

These changes are behind the FF ci_parallel_remote_includes (#413770 (closed))

How to set up and validate locally

1. Enable the feature flag;

   Feature.enable(:ci_parallel_remote_includes)

2. Run a basic web server to test slow includes.

   require 'rack'
   
   def run(env)
     puts "URL: #{env['REQUEST_URI']} started"
     sleep 3
     puts "URL: #{env['REQUEST_URI']} finished"
     "
     test:
       script: echo 'hello world'
     "
   end
   
   app = Proc.new do |env|
     [
       200,
       { "Content-Type" => "text/x-yaml" },
       [run(env)]
     ]
   end
   
   Rack::Server.start(app: app, Port: 9292)

3. Enable local includes on the Admin page http://gdk.test:3000/admin/application_settings/network
4. Use a basic CI config;

   include:
     - remote: http://0.0.0.0:9292/file1.yml
     - remote: http://0.0.0.0:9292/file2.yml
     - remote: http://0.0.0.0:9292/file3.yml
     - remote: http://0.0.0.0:9292/file4.yml
     - remote: http://0.0.0.0:9292/file5.yml
   
   build:
     script: exit 0

5. Result;

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.