Feature: Generic admission webhook

[lbernick]

/area API
/kind feature

We use the defaulting and validating admission webhook helpers downstream in Tekton and they are super useful. It would be nice to have a function for a generic admission webhook that takes care of certificates and reconciliation, but exposes the function func (ac *reconciler) Admit(ctx context.Context, request *admissionv1.AdmissionRequest) (resp *admissionv1.AdmissionResponse).

I'm trying to patch a CRD on creation, and have currently implemented this as a callback passed to defaulting.NewAdmissionController, but I realized the CRD namespace is not necessarily available in the unstructured object created from AdmissionRequest.Object and AdmissionRequest.OldObject, so I'm hoping to get this info from AdmissionRequest.Namespace.

I also want to avoid calling SetDefaults.

If this feature is of interest I can give implementing it a try.

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[dprotaso]

Related: #2656

I think having access the the AdmissionRequest on the context would help

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[lbernick]

/remove-lifecycle stale

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This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
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[lbernick]

/remove-lifecycle stale

Another use case for this is to capture UserInfo for the identity that submitted the AdmissionRequest, as described in tektoncd/pipeline#7068

[wlynch]

@lbernick UserInfo should be available via apis.GetUserInfo! 😎

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