Core: Use hpagent for proxies

[clemyan]

What's the problem this PR addresses?

Proxying with the tunnel package is... finnicky. I did not extensively investigate these but as a few examples of issues I encountered when I used it in the past:

• Does not use the specified CA certificates, even for HTTPS over HTTP proxy
• Does not use proxy after redirects

Fixes #6568
Fixes #5244

Hard to tell definitively without reproductions but these should be also fixed:

Fixes #3215
Fixes #2250

How did you fix it?

Use hpagent instead. This by itself fixes most HTTPS over HTTP proxying issues.

I have also reused HTTPS network settings (e.g. CA certs) for connecting to HTTPS proxies.

Side note: pem needs openssl installed to work which makes testing this (and running other HTTPS-related test) difficult in some cases (e.g. on Windows). Should we use another library for HTTPS testing?

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.