Add support for Tunnel Destination Groups

[cfinch]

Please add support for connecting to a host that is part of a Tunnel Destination Group, as documented here: https://cloud.google.com/iap/docs/tcp-by-host

Tunnel Destination Groups permit fine-grained access to VMs. Currently, to RDP to a host that is part of a Tunnel Destination Group, an end user must first use the gcloud command line tool to establish a tunnel. They can then use an RDP client to connect to the tunnel's port on localhost. This multi-step process isa nuisance for everyone, and a barrier to access for people who are new to the GCP ecosystem. It wold be great if IAP Desktop could also list Tunnel Destination Groups that the user can access.

[erkolson]

+1, this would be helpful