Dependency on heptiolabs/healthcheck prevents custom handler paths

[evillgenius75]

https://github.com/jetstack/kube-oidc-proxy/blob/d7bef6369c58185f5a66f62a49bffb6457f9a452/pkg/probe/probe.go#L12

This go package has not been contributed to in over 3 years and many customers are integrating tools to verify health checks for services and have standardized on /readyz /livez and the now deprecated /healthz in many cases (Ref: https://kubernetes.io/docs/reference/using-api/health-checks/ )

There has been a PR waiting in the Heptio repo for over a year that would allow for a custom path handler but no one is looking at PRs (REF: https://github.com/heptiolabs/healthcheck/pull/25#issuecomment-677710708)