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Use a dynamic RESTMapper for admission plugins
**What this PR does / why we need it**:
This change updates the REST mapper used by all admission plugins to be backed by cached discovery information. This cache is updated every ten seconds via a post start hook and will not attempt to update on calls to `RESTMapping`. It solely relies on the hook to keep the cache in sync with discovery.
This prevents issues with the `OwnerReferencesPermissionEnforcement` admission plugin when it is used with custom resources that set `blockOwnerDeletion`.
**Which issue(s) this PR fixes**:
`Fixes #...`
**Special notes for your reviewer**:
There are probably other ways the post start hook could be wired.
**Release note**:
```release-note
NONE
```
Signed-off-by: Monis Khan <mkhan@redhat.com>
@kubernetes/sig-api-machinery-misc
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