Use a `BROWSER` from the environment for interactive login to support VSCode Remote-SSH development

[spockz]

Feature description

We are using Visual Studio code on VMs to do our cloud development. Recently we switched from using SSH auth to using OAuth and GCM. This works fine on our local workstations as they have a browser. It doesn't work on gcm because it requires an active Desktop Session (e.g. CanUseEmbeddedWebView|CanUseSystemWebView). az login is able to login using the clients browser because it utilises the script managed by VSCode (and listed in the BROWSER env var) to open a browser and interact with it.

GCM could support a similar mechanism, potentially only on Linux(?), to reach out to Azure.

[dscho]

@spockz you seem to be already half-way through to open a PR, a PR I expect to be accepted very quickly. How about it? Give it a try?