Patch to make the HTTPClient proxy-aware

[wltbenade]

This patch addresses #2577

• Simple implementation to allow the HTTPClient to read proxy variables, if available, from the environment.
• Not sure where this should be documented.
• No tests updated, as all scalers should behave as normal.

Provide a description of what has been changed

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• Tests have been added
• 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)
• Changelog has been updated and is aligned with our changelog requirements

Fixes #

[JorTurFer]

/run-e2e
Update: You can check the progres here

[JorTurFer]

LGTM!
Thanks for this improvement! ❤️

Let's wait till e2e tests finish. Apart from this, could you create a PR in keda-docs adding information about this? Maybe in the section operate, you could create another subsection like HTTP Timeouts explaining how to configure this.
Do you think that those environment variables should be added also to the helm chart?

[JorTurFer]

We also need that you sign all the commits that you do. This link explains how to solve with already created commits

[wltbenade]

Thanks, @JorTurFer! Appreciate the quick response :)

I'll resubmit the PR with signed commits, and submit a PR to keda-docs.