Support stock multi-mode engines in configurations

[Starstrider42]

It would be useful if the TestFlight configuration field could apply to only one mode of a stock multi-mode engine (i.e., a part containing a MultiModeEngine module). Currently, the only way to have different behaviors for different modes is to tie TestFlightReliability_EngineCycle or the engine failure modules to a specific EngineID, but this does not allow different modes to have their own tech transfers or dynamic pressure/heat modifiers. Making modes supported at the configuration level would also simplify the part configs for behaviors that can already be supported with the current system, such as giving different modes different failure rates.

I think this is a matter of writing a suitable accessor for the active mode, and registering it in TestFlightInterop, but I haven't tried it yet.