Access control / semaphores

Describe your feature request

While thinking some more about cases where servers get stuck on external resources like #1796, I realized that you don't necessarily need a connection pool to limit access, some kind of semaphore would be enough.

Not sure how this would look like exactly, maybe something like this:

frankenphp_guard (
  key: 'slow-api-request',
  block_count: 10, # block on more than 10 parallel requests
  reject_count: 20, # throw on more than 20
  callback: function(){
    # slow IO to external API
  }
);

Doing this with files/apcu is actually pretty hard, mainly due to fatal errors and resulting deadlocks.
It would be easy to do on the FrankenPHP since we have full control over all threads.

Start with the external-resource scenario referenced in #1796 and the proposed frankenphp_guard entry point. The issue names no files or tests, so first clarify the API and semaphore behavior, including blocking and rejection thresholds. Done means the design and implementation scope are agreed for FrankenPHP's server threads.