feat: use env variable for setting a default tailnet id when using a system admin key

internal/config/funcs.go

@@ -25,6 +25,18 @@ func GetString(key, defaultValue string) string {
 	return defaultValue
 }
 
+func GetUint64(key string, defaultValue uint64) uint64 {
+	v := os.Getenv(key)
+	if v != "" {
+		vi, err := strconv.ParseUint(v, 10, 64)
+		if err != nil {
+			return defaultValue
+		}
+		return vi
+	}
+	return defaultValue
+}
+
 func validatePublicAddr(addr string) (*url.URL, string, int, error) {
 	scheme := "https"