Add a lane midline longitudinal distance offset option to RelativeLanePosition

Describe the feature

As a Scenario Author, I would like to be able to define positions relative to a vehicle's lane where a fixed longitudinal offset is specified along the entity's lane's midline and across road and road section boundaries it is clear where this offset lies so that I can describe distance cut-in and cut-out Scenarios with tight control over the longitudinal distance at which actions are triggered.

For example "The position given 5m ahead of the ego's front bumper along the ego's current lane's midline, one lane to the left." is a useful position to ensure a cut-in has a well-defined distance between the ego and vehicle cutting into its lane.

Describe the solution you would like

RelativeLanePosition is extended with a field named dLaneS or similar that is mutually exclusive with dS, allowing the lane midline offset to be specified.

We would need to describe well in the user guide how lane midlines connect at standard linkage road connections and junctions, and what the simulator's behaviour should be where it doesn't.

Describe alternatives you have considered

RelativeLanePosition allows a longitudinal offset along the road's reference line. However, we cannot use this as any curvature in the road will result in lane midline distances changing relative to reference line distances.

Describe the backwards compatibility

This would be a new Position subtype or a new field on RelativeLanePosition. No impact on backwards compatibility.

Additional context

A simple example of how the midline distance will differ from the road reference line distance with a curved road: