Support the "ephemeralcontainers" subresource in edit, get, patch, and replace

[dlipovetsky]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Allows kubectl users to use commands with the ephemeralcontainers subresource.

For example, kubectl debug does not allow a user to patch the ephemeral container's security context. An alternative is to use kubectl patch, e..g.: As of 1.27, kubectl debug allows the user to use "profiles" to set the ephemeral container's security context. But kubectl patch is useful for additional customization of ephemeral containers, and that's enabled by this PR.

kubectl patch pod/example --subresource=ephemeralcontainers --patch '{"spec":{"ephemeralContainers":[{"image":"docker.io/busybox:1.36.0","name":"test","command":["sleep"
],"args":["infinity"],"tty":true,"stdin":true,"securityContext":{"capabilities":{"add":["SYS_PTRACE"]}},"targetContainerName":"example"}]}}'

The above example works with this PR, but fails in master.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

I'm not sure that the "supported subresources" constraint is necessary. If I can make a case for removing it, I'll file a separate PR.

Does this PR introduce a user-facing change?

Added the `ephemeralcontainers` subresource to the kubectl `edit`, `get`, `patch`, and `replace` commands. 

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

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: dlipovetsky
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[dlipovetsky]

/cc @verb

[dlipovetsky]

/test pull-kubernetes-e2e-kind

[k8s-triage-robot]

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/lifecycle stale

[dlipovetsky]

/remove-lifecycle stale

I've added this PR to the sig-CLI office hours agenda for this week.

[dlipovetsky]

From @soltysh in today's SIG CLI office hours: This PR needs to consider the KEP that introduced the --subresources flag (https://github.com/kubernetes/enhancements/tree/master/keps/sig-cli/2590-kubectl-subresource#summary). The PR makes sense if ephemeralcontainers is a subresource in the same way that status and scale are, and whether they are or not is not clear.

Also from today's SIG CLI office hours: @ardaguclu mentioned a proposed KEP (kubernetes/enhancements#4293) to provide "custom" profiles to kubectl debug. If the KEP is accepted, it likely satisfies the use case that motivates this PR. But this PR might be valid even if the KEP is implemented.

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all PRs.

This bot triages PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the PR is closed

You can:

• Mark this PR as fresh with /remove-lifecycle stale
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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all PRs.

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• After 90d of inactivity, lifecycle/stale is applied
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/lifecycle rotten