Extend FollowTrajectoryAction with an initial path distance offset.

Describe the feature

Through use of the FollowTrajectoryAction and the position FollowingMode, often we (Five) want to re-use the same trajectory from our trajectory catalogue with multiple entities in the scene to avoid duplication and to create simple scenarios with e.g. one vehicle following another on the same trajectory.

However, to re-use the trajectory for a vehicle following another, we want to set an initial distance offset on the trajectory, rather than defining a timing difference as timing would require some clunky calculation by the user.

Describe the solution you would like

Add an optional initialDistanceOffset to FollowTrajectoryAction. This field defines a metres offset that the entity is either positioned or - in the case of the follow mode - where to virtually truncate the trajectory too. To a simulator, this trajectory would appear as

Describe alternatives you have considered

It is possible to calculate this distance offset with time, but it requires clunky calculations for the user.

I can't see a similar offset for Routes, but they would lack the precision we are looking for anyway.

Describe the backwards compatibility

The value is optional and defaults to the current behaviour (0m from start of path). Fully backwards compatible.