Add an upgrade integration test to confirm upgrades can succeed when a binary download proxy is configured

[cmacknz]

Relates:

• Fall back PGP download from Fleet server ignores Fleet server proxy configuration #4241
• Verifier does not honor proxy settings #4237

We have unit tests covering the underlying functionality now that #4241 is resolved, but in the process of fixing that issue we uncovered another problem with the fallback GPG URL using the wrong proxy in #4241

We need to add an integration test that confirms an upgrade can succeed when a binary download proxy is configured. We should have all the building blocks to do this since we already have tests that ensure Fleet proxy configuration works correctly. https://github.com/elastic/elastic-agent/blob/main/testing/integration/proxy_url_test.go

We need to extend those tests to cover the proxy for the upgrade download URL. That proxy URL was not respected for signature verification and this problem was able to hide in the code for a long time because this test is missing.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)