Download proxy URL on install

[CruzMarcio]

Please add a command line argument to install and enroll that allows us to set the download.proxy_url setting. On our company the fleet and elasticsearch servers are internal so it does not need a proxy, only to download from artifacts.elastic.co. When using the current --proxy_url parameter it sets the proxy for all.

[cmacknz]

Fleet now allows configuring the download proxy directly in the UI. https://www.elastic.co/guide/en/fleet/current/fleet-agent-proxy-managed.html

This uncovered a bug in this functionality that was fixed in 8.12.2 #4237