Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
refactor aggregator api group install
**What this PR does / why we need it**:
refactor aggregator apigroup install. move NewRESTStorage to
`staging/src/k8s.io/kube-aggregator/pkg/registry/apiservice/rest/storage_apiservice.go`
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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