Automatic merge from submit-queue
ImagePolicyWebhook Admission Controller
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**What this PR does / why we need it**: This is an implementation of the [image provenance proposal](https://github.com/kubernetes/kubernetes/blob/master/docs/proposals/image-provenance.md). It also includes the API definitions by @Q-Lee from https://github.com/kubernetes/kubernetes/pull/30241
**Special notes for your reviewer**:
Please note that this is the first admission controller to make use of the admission controller config file (`--admission-controller-config-file`). I have defined a format for it but we may want to double check it's adequate for future use cases as well.
The format defined is:
```
{
"imagePolicy": {
"kubeConfigFile": "path/to/kubeconfig/for/backend",
"allowTTL": 50, # time in s to cache approval
"denyTTL": 50, # time in s to cache denial
"retryBackoff": 500, # time in ms to wait between retries
"defaultAllow": true # determines behavior if the webhook backend fails
}
}
```
(or yaml)
**Release note**:
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```release-note
Adding ImagePolicyWebhook admission controller.
```
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