The agentsdk currently does a remap of the DERP map to change the
EmbeddedRelay node's URL to match the agent's access URL.
This PR makes changes to the `workspacesdk` (used by clients like the
CLI) and `vpn` (used by Coder Desktop) to match this behavior.
This enables us the ability to try Coder clients in dogfood over a VPN
without changing the global access URL.
Adds a new hidden subcommand `coder connect exists <hostname>` that checks if the name exists via Coder Connect. This will be used in SSH config to match only if Coder Connect is unavailable for the hostname in question, so that the SSH client will directly dial the workspace over an existing Coder Connect tunnel.
Also refactors the way we inject a test DNS resolver into the lookup functions so that we can test from outside the `workspacesdk` package.
Currently, importing `codersdk` just to interact with the API requires
importing tailscale, which causes builds to fail unless manually using
our fork.
Dean Sheather