[responseOps] switch proxy implementation from http(s)-proxy-agent to hpagent

[pmuellr]

Part of meta issue [ResponseOps] meta - improve proxy support #138319.

Originally this issue was titled upgrade http-proxy-agent from 2.x to latest 5.x, however since written, we've found the http/https agents available in the hpagent package seem to work consistently better than the http-proxy-agent / https-proxy-agent packages.

So, we think migrating to that package is the "upgrade" we'll want to do now.

From a POC in #131971, the amount of work is fairly small, as they have similar interfaces. However, there is missing support in hpagent right now that will need to be available before we can start using it: currently it is not possible to pass TLS options to the proxy. A POC PR is available adding that support in delvedor/hpagent#81

Assuming the new, incomplete proxy tests get added in #138071, we should complete those tests by running the additional scenarios that are currently failing with the existing agents.

original summary below:

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While looking into a responseOps issue with a proxy, I realized our http-proxy-agent package is really old. We're using 2.1.0 from 2018. The latest is 5.0.0 from 2021. We're already using the latest https-proxy-agent at version 5.0.0.

The main reason we're looking at this is that the newer version seems to support some TLS options which the older version doesn't, which may be causing us some issues when proxying https traffic over an http proxy - this has been a problematic scenario for some users.

This package is primarily used by the actions plugin, but also sees use in src/cli_plugin/install/downloaders and fleet. Just search the codebase for http-proxy-agent - only 19 hits. Because it's mainly actions, initially setting Team to ResponseOps

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)