[Elastic Agent] Ensure all processes work with `HTTP_PROXY` variables

[ruflin]

Proxy can be configured for Elastic Agent in 3 different ways:

• Command line flag --proxy-url
• Config option
• Environment variables HTTP_PROXY, HTTPS_PROXY

Elastic Agent over time will run new subprocesses. In https://github.com/elastic/beats/issues/25021 the expected behaviour was discussed. This issue is about discussing how we ensure / test that all processes we run support all the different options required.

[botelastic[bot]]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!