backend: Initial support for session proxy

This patch adds support for using the current session's HTTP and HTTPS
proxy from gio's ProxyResolver. For each type of proxy, it only uses the
first url because reqwest only supports that. We reuse the client
connection pool until it detects any proxy change.
4 jobs from session-proxy in 26 minutes and 40 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #229452
check

00:00:22

 
  Test
passed #229453
flatpak

00:22:48

 
  Review
passed #229454
review

00:03:28

manual #229455
allowed to fail manual
stop_review