Add support for Proxy

[fioan89]

Coder plugin uses behind the scenes an http client to connect to Coder deployments. The http cllient needs tp support proxy configurations so that requests can be routed through a specified proxy server.

Toolbox allows user to configure their proxy, and it also provides a service that exposes the values.

[matifali]

I guess we only need to verify with a Coder deployment running behind a proxy if that works with built-in ToolBox settings.