Custom download settings are not used by managed elastic-agent

[pchila]

Elastic agent does not use custom Agent Binary Download settings set in Fleet policy.

How to reproduce:

• Create a non-existing Proxy setting in Fleet (the URL is not important, just check that there's nothing listening for that URL)
  
• Create a new Agent Binary Download entry using the non-existing Proxy created at the previous step
  
• Create a Fleet policy that uses the new Agent Binary Download entry
  
• Trigger an upgrade from one of the affected version (tested this on 9.1.4 linux/amd64 elastic-agent) and see the upgrade succeeding even though the configured proxy is not there. The expected behavior is for the upgrade to fail since the packages cannot be downloaded using a non-existing proxy

This bug has been introduced with #9122: in that PR the following behavior has been implemented

• artifactDownloader is created only once when the Upgrader is instantiated at application startup
• artifactDownloader takes a pointer to the download settings at creation and then uses it for downloading packages
• before the PR, Upgrader would refresh its own settings and use the latest version of those settings to download

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[cmacknz]

Cross posting affected versions from the support investigation:

• 8.18 backport commit is included in v8.18.8, v8.18.7
• 8.19 backport commit is included in v8.19.6..v8.19.4
• 9.0 backport commit is included in v9.0.8, v9.0.7
• 9.1 backport commit in included in v9.1.6..v9.1.4
• main commit included in v9.2.0