[Fleet] Allow HTTP proxy_urls to be configured for all network connections that can be proxied.

[cmacknz]

Quoting an internal bug report:

I'm setting the Elastic Agent proxy URL for the downloads from https://artifacts.elastic.co/, but keep getting an error when triggering the Agent upgrade via Kibana (it fails to download). However, when I run a wget manually via CLI on the agent host, it is able to download via proxy. The only difference I noticed is on my wget I don't put "https://" in front of the https_proxy setting, but Fleet doesn't let me set a proxy URL without the "https://".
Is there any way to bypass this format restriction? Or am I missing something else?

An HTTPS URL will only work with a proxy that supports the CONNECT method, which uses the proxy like a TCP proxy that can’t look at the request content at all. It is probably also possible to setup a proxy to be able to man in the middle the connection by having it return a certificate signed by a CA the client already trusts but this is not common.

If a proxy doesn’t support the CONNECT method, then you need an HTTP URL to disable TLS. Fleet should support using http proxy_url configurations or there’s no other way to make this work.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)