feat: validate if InPlacePodVerticalScaling feature gate is enabled

[mikouaj]

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Description

The InPlacePodVerticalScaling feature gate is crucial for the Kube Startup CPU Boost operator. Without this feature gate enabled, the operator will increase POD resources without the possibility to revert them back when needed. Therefore, the operator should validate if the InPlacePodVerticalScaling is enabled.

NOTE: this is interim solution until InPlacePodVerticalScaling graduates to beta.
There is no perfect way to programmatically validate if a feature gate is enabled cluster wide. The most straightforward but not perfect solution is to use /metrics/ API that eventually consistent.

References

• Error: pod resources reversion failed #62