Support for defaulting of CR fields via mutating admission webhooks

[raffaelespazzoli]

Feature Request

Some CRs need a way to set some fields to default values.
Please add a facility to the operator-sdk to manage this situation. From a developer perspective a stub field defaulting method should be generated and the developer needs to fill it up. Requesting the generation the generation of this method should be optional for the operator-sdk generate controller command

Is your feature request related to a problem? Please describe.
As I said some CRs require defaulting of fields, as of now every developer of an operator has to come up with its own solution.

Describe the solution you'd like
I believe there are multiple ways to implement this, but perhaps the most kubernetes-pure would be via a MutatingAdmissionController. The generated operator could provide a http endpoint to listen for these MutatingAdmissionController webhook calls. Regardless the user experience should be the one defined above.

[hasbro17]

As discussed in #1217 (comment) we'll be using the upstream controller-runtime APIs for mutating admission webhooks.
Example:
https://github.com/kubernetes-sigs/controller-runtime/blob/5e3a6da770e2f16f8ce9b0905eaa74974fe7a4c0/examples/builtins/mutatingwebhook.go

We just need to add the scaffolding,docs and examples.

[openshift-bot]

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[hasbro17]

/remove-lifecycle stale

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/lifecycle stale

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[openshift-ci-robot]

@openshift-bot: Closing this issue.

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