Automatic merge from submit-queue (batch tested with PRs 60208, 60084, 60183, 59713, 60096). 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>.

Add external metric type to HPA API

**What this PR does / why we need it**:
Add external metric type to HPA API proposed in xref https://github.com/kubernetes/community/pull/1801

**Release note**:
```release-note
Allows HorizontalPodAutoscaler to use global metrics not associated with any Kubernetes object (for example metrics from a hoster service running outside of Kubernetes cluster).
```