Feature Description

kcp has an API resource called APIConversion. This used to be working as a way for API providers to define API version conversion rules (something that is usually done via webhooks in upstream Kubernetes). The original PR implementing this was #2105, but its functionality got removed with #3017 as it caused issues with the Kubernetes rebase.

We should bring the feature back, as it is critical to API providers that want to evolve their API to another version.

Proposed Solution

We should re-implement APIConversions (it would make a lot of sense to look at the previous implementation). There were a couple of potential concerns raised with the original implementation (also see Slack thread linked below):

it made rebase harder at the time.

I think converting lists was hard or impossible.

An implementation should include documentation for this feature and e2e testing. Also look for older issues like #2696 to see potential improvements.

Alternative Solutions

No response

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Additional Context

See this Slack thread for some recent discussion.