add activation feature for cpu & memory scaler

[kunwooy]

Currently, the cpu & memory scalers lack the activation feature because they delegate 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.

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 our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• 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 #

[semgrep-app]

Semgrep found 2 context-todo findings:

• pkg/scalers/cpu_memory_scaler_test.go
  • L165 - Triage
  • L179 - Triage

Consider to use well-defined context

_{Ignore this finding from context-todo.}

[kunwooy]

Semgrep found 2 context-todo findings:

• pkg/scalers/cpu_memory_scaler_test.go

  • L165 - Triage
  • L179 - Triage

Consider to use well-defined context

Ignore this finding from context-todo.

This is resolved

[semgrep-app]

Semgrep found 1 wrong-wrapped-error finding:

• pkg/scalers/cpu_memory_scaler_test.go
  • L166 - Triage

Semgrep found a possible error wrong wrapped.

_{Ignore this finding from wrong-wrapped-error.}

[semgrep-app]

Semgrep found 1 wrong-wrapped-error finding:

• pkg/scalers/cpu_memory_scaler_test.go
  • L167 - Triage

Semgrep found a possible error wrong wrapped.

_{Ignore this finding from wrong-wrapped-error.}