Support custom HTTP proxy

[geekifier]

Many of us have to use various VPN solution in order to work, some of which prevent proper connections into locally-hosted HA, especially if it's on another VLAN.

It would be nice to have the ability to set a custom HTTP proxy for use with the companion app on MacOS, so that it can be set separately from whatever corporate proxy might be getting set at the system level.

I also tried settings this with an http_proxy ENV var, but it did not seem to work.