This MR changes exilic uses of ACTS VectorMultiTrajectory as backend for TrackContainer (and other uses in TrackFindingTool).
It is a followup of !63302 (closed)
I guess the name TrackStateBackend can be debated (this was Paul's suggestion).
To be able to roll in the Athena version of it I think we would benefit from an additional macro. i.e. to able to consistently switch to to it in several places at once.