Provide model info via dataRef to 3rd party plugins when having AI control

[TwinFan]

Current Situation / Problem
3rd party plugins cannot find out which model a rendered plane uses.
There is a use case at least for the camera plugin ABC, which asks for a value similar to what LTAPI returns through LTAPIAircraft::getCslModel().

Suggested Solution

Extend the already existing set of shared dataRefs:

• Add a shared dataRef sim/multiplayer/position/plane#_cslModel filled with XPMP2::Aircraft::GetModelName()
• Document the new shared dataRef in the developer docs

Benefits
Use cases are probably limited...but at least ABC has what it asks for. It somehow uses it to fine-tune the camera per displayed model.

Additional context
Discussed in a support thread in the context of the Remote Client. The solution will be generic for XPMP2, however, ie. for any plugin based on XPMP2.