Thanks for doing this! Ideally we'd need end-to-end tests to make sure that it works as expected with the actual admission webhook deployed (similar to the testInvalidRulesAreRejected() test). We also need to update the configuration + doc of the admission webhook.

If you feel that it's over your knowledge, we can defer this to a follow-up PR. I'll test locally anyway.

@simonpasquier Let me try implementing first. I'll let you know if these items have to be deferred to the new PRs. Thank you for your suggestion!

@simonpasquier I updated E2E test and document already.

Sorry for the back-and-forth on this PR, my brain is slow to process apparently. Shouldn't we harden the validation of the pagerdutyUrl field and change the type from *string to *URL?

To give a bit more context, the use case for admission webhooks is to supplement validation which can't be implemented via kubebuilder markers and/or CEL. I'm not saying that we don't need an admission webhook service for Alertmanager CRD but I'd prefer to avoid maintaining it unless it's absolutely required :)

@simonpasquier I do agree. It makes more sense to use kube validation instead of maintaining the code in the webhook.

Let me close this PR and do what you suggest instead. Thank you!