Add health and readiness checks to `eirini-controller`

[gcapizzi]

eirini-controller should expose health endpoints as described here. According to that document, healthz is deprecated in favour of livez and readyz, so we should have:

• two endpoints, one for livez and one for readyz, similar to these
• two probes configured in our Deployment, a livenessProbe and a readinessProbe, like this