feat: add activation feature for CPU/Memory scaler

[kunwooy]

Currently, the cpu & memory scaler lacks the activation feature because it delegates the scaling responsibilities to the built-in Kubernetes HPA controller. As a result, even if the scale target is currently scaled-out by the cpu/memory metric being above the threshold value, if some other Keda scalers which use External Metrics are used in conjunction with the cpu/memory scaler, it will be deactivated (and thus scaled to zero) when all other scalers using External Metrics are deactivated.

Hence, my proposal is to introduce a way to check the activation of the cpu/memory scaler. Since the scaling behavior will be handled by the HPA controller, cpu/memory scaler only needs to feed in the activation value to the scaled object controller in its GetMetricsAndActivity() method. Moreover to enable such feature, I introduce activationValue field in cpu/memory trigger's metadata.

(I have accidentally closed the last pull request: #6174)

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6057

Relates to #

[kunwooy]

@JorTurFer You suggested that I query the metrics server directly instead of querying HPA. I have modified the code (#6174 (comment)).

If approved, modification to the helm chart is needed since the operator's service account needs permission on pods.metrics.k8s.io APIs

[SpiritZhou]

Do you mind updating the CHANGELOG.md as well?

[kunwooy]

@SpiritZhou I updated the CHANGELOG.md too.