Add a new server Railtie block: - #39953

- This is similar to other railties blocks (such as `console`,
  `tasks` ...). The goal of this block is to allow the application
  or a railtie to load code after the server start.

  The use case can be to fire the webpack or react server in
  development or start some job worker like sidekiq or resque.

  Right now, all these tasks needs to be done in a separate
  shell and gem maintainer needs to add documentation on
  how to run their libraries if another program needs to run
  next to the Rails server.

  This feature can be used like this:

  ```ruby
    class SuperRailtie < Rails::Railtie
      server do
        WebpackServer.run
      end
    end
  ```