Need HUD element for fence status

Pilots currently have no situational awareness of the fence status. There is a FENCE_STATUS message, but as far as I can see there is no way to see that status while flying in MissionPlanner.
I suggest a new optional HUD element. It could be de-selected by defaults, and enabled with the HUD Items menu.
I think it should show "Fence" in 3 colors:
- grey for disabled
- green for OK
- red for fence breach

Alternatively, the color of the fence on the map could change based on FENCE_STATUS? Or dashed line for fence when disabled?

Start by tracing how the FENCE_STATUS message is exposed in MissionPlanner and how entries in the HUD Items menu are defined; no files or tests are named in the issue. Resolve whether the HUD element or map styling is the intended presentation, then verify that the chosen display is optional and distinguishes disabled, OK, and breach states by the agreed colors.

Written by the indexing model from the issue text.