kubernetes: Report event for the cases when probe returned Unknown result

There are a few cases when the Unknown result can be returned by the probe. For example see: https://github.com/kubernetes/kubernetes/issues/106682.

It will be useful to expose those in logs and as an event similar to what we do for Warning: https://github.com/kubernetes/kubernetes/blob/35f3fc59c1f29eebc8ff8705ecc3d4db7c4cbbc6/pkg/kubelet/prober/prober.go#L115

What needs to be done:

1. Implement logging and exposing event in case of Unknown probe result somewhere around https://github.com/kubernetes/kubernetes/blob/35f3fc59c1f29eebc8ff8705ecc3d4db7c4cbbc6/pkg/kubelet/prober/prober.go#L115
2. For each instance of probes potentially returning the Unknown result have a test that validates this test case. It may be either e2e test or unit test. You may find an example of e2e test that results in Unknown result from exec probe in one of the links from https://github.com/kubernetes/kubernetes/issues/106682

/sig node /help wanted /good-first-issue /priority backlog

This issue is marked as “good first issue”. Before assigning issue to yourself, please make sure you understand the assignment and list cases you will need to cover with tests.

cc @aojea