client/tailscale,cmd/{cli,get-authkey,k8s-operator}: set distinct User-Agents

This helps better distinguish what is generating activity to the
Tailscale public API.

Updates tailscale/corp#23838

Signed-off-by: Percy Wegmann <percy@tailscale.com>

cmd/get-authkey/main.go

@@ -51,6 +51,7 @@ func main() {
 
 	ctx := context.Background()
 	tsClient := tailscale.NewClient("-", nil)
+	tsClient.UserAgent = "tailscale-get-authkey"
 	tsClient.HTTPClient = credentials.Client(ctx)
 	tsClient.BaseURL = baseURL