Add healthcheck route

[winston0410]

At the moment, there is no healthcheck route for miniserve server, which is crucial for setting up a readiness probes and an liveness probe in K8S.

Using / as healthcheck alternative would not work, as we cannot set auth for healthcheck in K8S.

[svenstaro]

I agree this would be nice to have. Would you like to add this? Should be fairly straight-forward.