Support 'Scale' subresource for CRDs

[eloyekunle]

Backend implementation via k8s.io/client-go/scale currently unable to work with cluster-scoped resources. Tracking issue: kubernetes/kubernetes#81380

Closes #4167

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@b565ea3). Click here to learn what that means.
The diff coverage is 25.71%.

@@            Coverage Diff            @@
##             master    #4195   +/-   ##
=========================================
  Coverage          ?   45.95%           
=========================================
  Files             ?      198           
  Lines             ?     9298           
  Branches          ?      105           
=========================================
  Hits              ?     4273           
  Misses            ?     4765           
  Partials          ?      260

Impacted Files	Coverage Δ
...ackend/resource/customresourcedefinition/detail.go	0% <0%> (ø)
...ckend/resource/customresourcedefinition/objects.go	0% <0%> (ø)
src/app/backend/api/types.go	38.88% <0%> (ø)
src/app/backend/handler/apihandler.go	27.24% <100%> (ø)

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[eloyekunle]

This is ready for review, PTAL.

/assign @jeefy @maciaszczykm @floreks

[maciaszczykm]

Please rebase.

[eloyekunle]

Rebased.

[maciaszczykm]

Tested locally.

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: eloyekunle, maciaszczykm

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