Add /readyz and a useful /healthz

**What would you like to be added**:

The `/healthz` endpoint is pretty useless for anything other than verifying that the application is up.
```go
metricsMux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, "ok")
})

```

It would be helpful to have reliable handlers to support liveness and readiness checks.

**Why is this needed**:

I want to know when the webhook is functioning (can fetch the TLS secret, can talk to the k8s API) before it starts serving traffic.

Start by finding the Go setup for metricsMux and the existing /healthz handler shown in the issue. Trace how the webhook accesses the TLS secret and Kubernetes API. Done means /readyz and a useful /healthz reliably report whether the webhook is functioning and ready to serve traffic.

Written by the indexing model from the issue text.