dsh-trip-itinerary: Itinerary Planning

The `dsh-trip-itinerary` plugin provides one-click trip itinerary planning and is maintained by `uckkk`. With a pure Node code core and no network dependencies, it can precisely calculate itineraries based on parameters and return detailed results. Users can install it with the `dsh plugin add` command, then invoke its registered tools in a session to use it. The plugin runs with DSH process privileges; review the source code and MIT license before installation.