need a way to add a custom aux function

the nice Aux Function tab in MissionPlanner only displays aux functions that it has extracted from parameter meta data. If you are testing a pending new aux function it can't be triggered
We should be able to enter a number for the aux functions UI

Start by locating the Mission Planner Aux Function tab and reading how its available functions are populated from parameter metadata. The change is done when the UI accepts a numeric aux-function value for testing a function that is not yet present in the metadata and can trigger it.

Written by the indexing model from the issue text.