Feature Request: Add a plugin or capability similar to MagFit Tool

#### Issue details
An equivalent to MAVExplorer's Magfit Tool should be created. Mission Planner is a far better tool for the normal user to interact with.

https://ardupilot.org/copter/docs/common-magfit.html?highlight=mavexplorer#using-mavexplorer-s-integrated-magfit-utility

I think this would likely be a plugin at first. (I'm mostly creating an issue to remind myself to do have a try at a plugin later.)

Start by reviewing the linked ArduPilot MagFit documentation and comparing it with MAVExplorer’s integrated Magfit Tool. The issue names no Mission Planner files, tests, or plugin entry point; done means providing an equivalent capability for normal Mission Planner users, initially potentially as a plugin.

Written by the indexing model from the issue text.