Add all available annotations to k8s Backend

[ldez]

What does this PR do?

• add all available annotations
  • add passTLSCert annotations.
  • add error pages annotations.
  • add max conn annotations.
  • add rate limit annotations.
• enhance template readability

Motivation

Homogenization of the providers [part2]: all providers must have the same options available.

More

• Added/updated tests
• Added/updated documentation

Additional Notes

Several PRs will come after that for each provider.

Related to #618, #1465
Closes #2783

[timoreimann]

Here's one point where our homogenization might not be good: IMHO, Traefik health checks make no sense with Kubernetes since readiness probes are supposed to take care of this. There's a high chance that if both are enabled, users will be confused because the Endpoints controller may remove a pod from rotation while Traefik still wants to health-check it, leading to weird error messages.

@dtomcej @errm can you think of a use case where it makes sense to support both?

[mmatur]

LGTM 👏

[ldez]

ping @errm @dtomcej

[dtomcej]

@timoreimann @ldez If we are constructing backends by parsing service endpoints, then that endpoint list already takes into account pod readiness and liveliness probes. I agree that introducing an independent/redundant health check would be confusing and indeed cause lifecycle issues.

[dtomcej]

Outside of the h/c issue discussed,

LGTM

[ldez]

I removed the health check.
Now we have to define the annotation's names, before writing the documentation.