feature: API version migration with conversion webhook

[gman0]

Feature Description

Conversion migrations of APIExports are not documented, tests are missing.

• Where and how to run the conversion webhook
  • In respect to sharding
  • ^ These should follow the same conditions as with any other webhooks we support (VAP & MAP)
• What happens when the webhook is unavailable

Proposed Solution

We should document how an APIExport owner can migrate between versions of their schema

We have an e2e #4339 with a simple conv webhook scenario. Currently breaks things. We should find out exactly what's missing, and fix it.

Alternative Solutions

No response

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Additional Context

No response