Add support for proxy authentication

Coder Toolbox can proxy all the REST API requests. It honors the Proxy configuration done in the Toolbox Settings (not to be confused with Coder Toolbox Settings which are specific to Coder). However the API available in Toolbox does not expose the authentication details. An issue was logged on Toolbox: https://youtrack.jetbrains.com/issue/TBX-14532/Missing-proxy-authentication-settings
Once the issue is addressed we can finish the implementation on our side by reading the credentials provided by the new API and configure the okhttp proxy authenticator.