Automatic merge from submit-queue (batch tested with PRs 45993, 50293)
[Federation] HPA controller
This PR implements the design listed in https://github.com/kubernetes/community/pull/593.
This is still a work in progress, and needs more unit tests to be added.
I will add the integration tests and e2e tests in a separate PR(s).
@kubernetes/sig-federation-pr-reviews
**Release note**:
```
Horizontal Pod Autoscaling is now available as an alpha feature in federation.
It can be used to distribute and scale workload across clusters joined in a federation.
In its current form, it works only on cpu utilization and the support for other metrics is yet to be built in.
```
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