feat: Add agent authentication based on instance ID (#336)

* feat: Add agent authentication based on instance ID

Each cloud has it's own unique instance identity signatures, which
can be used for zero-token authentication. This change adds support
for tracking by "instance_id", and automatically authenticating
with Google Cloud.

* Add test for CLI

* Fix workspace agent request name

* Fix race with adding to wait group

* Fix name of instance identity token

.vscode/settings.json

@@ -36,11 +36,13 @@
     "gossh",
     "hashicorp",
     "httpmw",
+    "idtoken",
     "isatty",
     "Jobf",
     "kirsle",
     "manifoldco",
     "mattn",
+    "mitchellh",
     "moby",
     "nhooyr",
     "nolint",