Improve/adapt Knative defaulting and validation for K8s object types

[ReToCode]

Problem description

Kubernetes has a lot of defaulting and validation implemented for its own types, e.g. https://github.com/kubernetes/kubernetes/blob/master/pkg/apis/core/v1/zz_generated.defaults.go#L305 for a container. Knative implements its own defaulting like here. This is useful for stuff where Knative has other defaults or additional values.

This works in most cases, but we loose a lot of defaulting that K8s would do and have to reimplement, e.g. with probes: https://github.com/knative/serving/blob/main/pkg/apis/serving/v1/revision_defaults.go#L157

Also it causes issues like #14771 and we are lacking to default fields that are newly introduced in K8s like StartupProbes.

So it might be good to think about re-using K8s defaulting and validation doing the Knative stuff afterwards.

This is also related to the discussion in: #13204

[ReToCode]

cc @skonto @dprotaso

[ReToCode]

See also discussion in #14853 (comment)

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