Automatic merge from submit-queue (batch tested with PRs 49989, 49806, 49649, 49412, 49512)
This adds an etcd health check endpoint to kube-apiserver
addressing https://github.com/kubernetes/kubernetes/issues/48215.
**What this PR does / why we need it**:
This ensures kube-apiserver `/healthz` endpoint fails whenever connectivity cannot be established to etcd, also ensures the etcd preflight checks works with unix sockets
**Which issue this PR fixes**: fixes#48215
**Special notes for your reviewer**:
This PR does not use the etcd client directly as the client object is wrapped behind the storage interface and not exposed directly for use, so I decided to reuse what's being done in the preflight. So this will only check fail for connectivity and not etcd auth related problems. I did not write tests for the endpoint because I couldn't find examples that I could follow for writing tests for healthz related endpoints, I'll be willing to write those tests if someone can point me at a relevant one.
**Release note**:
```release-note
Add etcd connectivity endpoint to healthz
```
@deads2k please help review, thanks!
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