licenses: add GitHub Action to update license notices (#5433)

This will update a licenses/tailscale.md file with all of our go
dependencies and their respective licenses. Notices for other clients
will be triggered by similar actions in other repos.

Co-authored-by: Andrew Dunham <andrew@tailscale.com>
Signed-off-by: Will Norris <will@tailscale.com>
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

.github/licenses.tmpl

@@ -0,0 +1,17 @@
+# Tailscale CLI and daemon dependencies
+
+The following open source dependencies are used to build the [tailscale][] and
+[tailscaled][] commands. These are primarily used on Linux and BSD variants as
+well as an [option for macOS][].
+
+[tailscale]: https://pkg.go.dev/tailscale.com/cmd/tailscale
+[tailscaled]: https://pkg.go.dev/tailscale.com/cmd/tailscaled
+[option for macOS]: https://tailscale.com/kb/1065/macos-variants/
+
+## Go Packages
+
+Some packages may only be included on certain architectures or operating systems.
+
+{{ range . }}
+ - [{{.Name}}](https://pkg.go.dev/{{.Name}}) ([{{.LicenseName}}]({{.LicenseURL}}))
+{{- end }}