Draft: Implement parallel execution for CI includes

What does this MR do and why?

This MR implements a parallel execution process for CI includes.

• Related to #296532 (closed)
• I used the concurrent-ruby gem, which is installed in our project via a couple of gems. However, we should add this gem individually after making a decision about this MR.
• Documentation for Concurrent::Promise: https://ruby-concurrency.github.io/concurrent-ruby/1.1.5/Concurrent/Promise.html
• It's behind a feature flag ci_parallel_include_fetching.

Screenshots or screen recordings

A simple example;

promises = objects.map do |object|
  Concurrent::Promise.new { sleep 5 }.execute
end

results = promises.map do |promise|
  promise.wait

  if promise.fulfilled?
    promise.value
  else
    raise promise.reason
  end
end

Or with the new Module;

Gitlab::Parallel.execute(objects) do |object|
  sleep 5
end

How to set up and validate locally

1. Create a couple of TCP server to imitate remote files. (to experience a static delay time)

require 'socket'

server = TCPServer.new 5676 # we need a couple of ports

while session = server.accept
  request = session.gets
  puts request

  sleep 5 # delay time

  session.print "HTTP/1.1 200\r\n"
  session.print "Content-Type: text/html\r\n"
  session.print "\r\n"
  session.print "Hello world! The time is #{Time.now}"

  session.close
end

2. Disable validate_local_request in lib/gitlab/url_blocker.rb to be able to use local fake includes.
3. Open http://gdk.test:3000/-/graphql-explorer
4. Enter this config;

query getCiConfigData($projectPath: ID!, $sha: String, $content: String!) {
  ciConfig(projectPath: $projectPath, sha: $sha, content: $content) {
    errors
    mergedYaml
    status
  }
}

5. With these query variables;

{
  "projectPath": "root/project-path",
  "sha": "latest sha",
  "content": "include:\n  - remote: 'http://localhost:5676/a.yml'\n  - remote: 'http://localhost:5677/b.yml'\n  - remote: 'http://localhost:5678/c.yml'\n  - remote: 'http://localhost:5679/d.yml'\n"
}

6. Compare the response time with and without Feature.enable(:ci_parallel_include_fetching).

Without ci_parallel_include_fetching;

With ci_parallel_include_fetching

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.