Automatic merge from submit-queue (batch tested with PRs 63793, 65989). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Enable kubectl proxy to set tcp keepalive
**What this PR does / why we need it**:
Allows setting keepalive period for kubectl proxy.
Fixes#63727
**Special notes for your reviewer**:
/assign @brendandburns
**Release note**:
```release-note
Introduce a new flag `--keepalive` for kubectl proxy to allow setting keep-alive period for long-running request.
```
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