feat: unify php_server logic

[AlliBalliBaba]

Currently the concept of a php_server only exists on the caddy side and not the FrankenPHP side.
Lately we have been moving more and more in a direction of scoping requests or workers to specific php_server blocks.

This PR is an attempt at refactoring the current php_server logic so it is properly mirrored on the FrankenPHP side without BC breaks for library users (and to prevent future bugs like mentioned in #2487)

[AlliBalliBaba]

I changed the api again a bit, this feels like it makes the most sense (no idx passing required):

server := frankenphp.NewServer(root, env, splitPath)

frankenphp.Init(frankenphp.WithServer(server))

server.ServeHTTP(request, responseWriter)

[AlliBalliBaba]

Another subtlety I fixed: the "external workers" from RegisterWorkers should probably persist across reloads.

[henderkes]

I think there's also a problem with DrainWorkers on main at the moment. Seems like it takes the same path with drainPHPThreads as Shutdown logic, but then never sets the state so the php threads could restart. Not directly related to this PR but probably better to fix it right here (or remove the function, it's marked experimental anyways).

[AlliBalliBaba]

DrainWorkers always worked this way. I think in the past it was a way for library users to have a graceful worker shutdown. But it is not necessary anymore. IMO we can also remove it

[henderkes]

+1 for removal because it currently hard-kills the current php runtime with no way to recover it, instead of letting threads perform a graceful reload.