consider making firestorm dependency of actix-router optional

[matklad]

I've noticed firestorm dependency in Cargo.lock of my application. This dependency comes from actix-router. My understanding is that this is a dev-tool used for internal profiling of actix-router.

I understand that it is a no-op crate by default, but I still would prefer not to have this entry in my Cargo.lock to avoid worrying about supply chain attacks and spending time on figuring out why this relatively unknown crate is there.