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

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