[WIP] apiserver: wire FlushError for ResponseWriter

[tkashem]

What type of PR is this?

Add support for the new FlushError method to the ResponseWriter object.

/kind feature

What this PR does / why we need it:

http.Flusher does not return error

type Flusher interface {
	// Flush sends any buffered data to the client.
	Flush()
}

On the other hand, FlushError returns an error if flushing was not successful, the request handler can invoke FlushError and use the returned error to terminate

FlushError() error

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

- [KEP]: https://github.com/kubernetes/enhancements/pull/4462

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tkashem
Once this PR has been reviewed and has the lgtm label, please assign deads2k for approval. For more information see the Kubernetes Code Review Process.

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Needs approval from an approver in each of these files:

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• staging/src/k8s.io/apiserver/OWNERS
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[k8s-ci-robot]

@tkashem: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-e2e-kind	d78ce8e	link	true	/test pull-kubernetes-e2e-kind
pull-kubernetes-linter-hints	d78ce8e	link	false	/test pull-kubernetes-linter-hints
pull-kubernetes-integration	d78ce8e	link	true	/test pull-kubernetes-integration
pull-kubernetes-verify-lint	d78ce8e	link	true	/test pull-kubernetes-verify-lint
pull-kubernetes-verify	d78ce8e	link	true	/test pull-kubernetes-verify
pull-kubernetes-unit	d78ce8e	link	true	/test pull-kubernetes-unit
pull-kubernetes-node-e2e-containerd	d78ce8e	link	true	/test pull-kubernetes-node-e2e-containerd
pull-kubernetes-e2e-kind-ipv6	d78ce8e	link	true	/test pull-kubernetes-e2e-kind-ipv6
pull-kubernetes-e2e-gce	d78ce8e	link	true	/test pull-kubernetes-e2e-gce

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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