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]

The public api is now minimized to these 4 functions:

// server registration
func WithServer(idx int, resolvedDocumentRoot string,splitPath []string,env map[string]string) Option

// worker scoping
func WithWorkerServerScope(serverIdx int) WorkerOption
func WithWorkerMatcher(matcherFunc func(*http.Request) bool) WorkerOption

// request scoping, less overhead than WithWorkerRequestScope()
func ServeHTTPSrv(serverIdx int, responseWriter http.ResponseWriter, request *http.Request, opts ...RequestOption) error

This PR also removes the circular fc -> request dependency, which allows simplifying the contextHolder and saving some overhead.