UPSTREAM: <carry>: Keep system:cluster:* groups

[ntnn]

What type of PR is this?

See kcp-dev/kcp#3513

Two types of users get to keep groups starting with system:cluster:*:

1. Service Accounts
2. Authenticated users not originating from the cluster the request was made from

/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

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

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[mjudeikis]

Do we need to care if people can use these groups in their configuration? Do we have some safe-guards?

[ntnn]

Kube should protect system: users and groups by itself, for per-workspace auth I'm not so sure anymore after I've noticed that the system:authenticated group is missing.

At least we also have the ForbidSystemUsername for per-workspace auth: https://github.com/kcp-dev/kcp/blob/e23e1b842eef9d6850736764d24b17ffd34c4f88/pkg/authentication/usernames.go#L28
I think we'd need the same for ForbidSystemGroups and some tests on top.

[ntnn]

Ah, you can't assign groups directly via per-workspace auth. That's only happening by manually mapping it in the authentication configuration.
But we could and should protect the system:cluster: groups.

And we do: https://github.com/kcp-dev/kcp/blob/e23e1b842eef9d6850736764d24b17ffd34c4f88/pkg/authentication/index.go#L241-L245
And there's a test for it: https://github.com/kcp-dev/kcp/blob/e23e1b842eef9d6850736764d24b17ffd34c4f88/test/e2e/authentication/workspace_test.go#L209-L221
So it's all good