Ensure make update-mocks does not inject existing boilerplate

[embik]

What type of PR is this?

I noticed (while working on #3565) that make update-mocks always prepended the existing boilerplate header to generated fake clients, so I ended up with the header being injected a second time on existing files that already had it. This PR makes sure that the boilerplate prepend logic in the Makefile is only triggered if the file doesn't already start with the boilerplate.

After adding this and running said make target, it turned out that FakePublisherClient wasn't up-to-date, so I'm including that in the PR (we can also extract it if preferred).

/kind bug
/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

Hi @embik. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[cpanato]

/ok-to-test

[saschagrunert]

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: embik, saschagrunert

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