helm: Added support for HPA behavior

[styler]

Description

This pull request adds support for Horizontal Pod Autoscaler (HPA) behavior.

https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#configurable-scaling-behavior

Related Issues

List related issues.

Testing

Manual testing was conducted to ensure that the new HPA behavior was working as expected.

Checklist

• Does my change need to be backported to a previous release?

  • What backport versions were discussed with the Maintainers in the Issue?

• I made sure to update CHANGELOG.md.

  Remember, the CHANGELOG needs to mention:

  • Any new features
  • Any changes to our included version of Envoy
  • Any non-backward-compatible changes
  • Any deprecations

• This is unlikely to impact how Ambassador performs at scale.

  Remember, things that might have an impact at scale include:

  • Any significant changes in memory use that might require adjusting the memory limits
  • Any significant changes in CPU use that might require adjusting the CPU limits
  • Anything that might change how many replicas users should use
  • Changes that impact data-plane latency/scalability

• My change is adequately tested.

  Remember when considering testing:

  • Your change needs to be specifically covered by tests.
    • Tests need to cover all the states where your change is relevant: for example, if you add a behavior that can be enabled or disabled, you'll need tests that cover the enabled case and tests that cover the disabled case. It's not sufficient just to test with the behavior enabled.
  • You also need to make sure that the entire area being changed has adequate test coverage.
    • If existing tests don't actually cover the entire area being changed, add tests.
    • This applies even for aspects of the area that you're not changing – check the test coverage, and improve it if needed!
  • We should lean on the bulk of code being covered by unit tests, but...
  • ... an end-to-end test should cover the integration points

• I updated DEVELOPING.md with any any special dev tricks I had to use to work on this code efficiently.

• The changes in this PR have been reviewed for security concerns and adherence to security best practices.

[LanceEa]

@styler - thanks for the PR, would you mind writing up an issue first outlining the reasoning for wanting to add this.

Also, I took a quick glance at the link and it looks like this feature wasn't stable until 1.23 and we support back to 1.22 still so I just want to make sure we are handling this in backwards compatible way.

[styler]

@LanceEa For sure!

[NikitaCloudRuntime]

Hello
We're also interested in this feature :)
@styler did you have any chance you work on it and adding the issue mentioned in the comment above?

[LanceEa]

@styler - were you able to write up an issue on this topic? The key thing for me is outlining the plan for handling backwards compatibility since this API didn't become stable until 1.23 and we still maintain support back to 1.21.