Issue #3313404: Use symfony/runtime for less bespoke bootstrap/compatibility...

Issue #3313404: Use symfony/runtime for less bespoke bootstrap/compatibility with varied runtime environments

A custom runtime is provided which knows how to create the Drupal specific runner that will use the DrupalKernel. The Drupal specific runner is currently the same as Symfony's HttpKernelRunner. However, it provides us with a place to put runtime specific code such as Revolt's eventloop.