[WIP] Implement HVPA v2 proposal

[amshuman-kr]

What this PR does / why we need it:

Implement HVPA v2 proposal.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Based on #76 which can now be closed.

Release note:

Change API Group to `autoscaling.gardener.cloud`

The change of API group to `autoscaling.gardener.cloud` is a breaking change and existing Hvpa resources with the old API Group `autoscaling.k8s.io` will NOT be automatically migrated to the new API group.

Implement scale subresource - Now HPA, if deployed, will scale HVPA's scale subresource instead of the HVPA's target

The scaling is not based on HPA/VPA weightage anymore. Max of total resources recommended by HPA/VPA is considered for scaling based on provided scale intervals.

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[gardener-robot]

@amshuman-kr You need rebase this pull request with latest master branch. Please check.