fix: initial offset to be merged with target for lane offset action controller

This tries to fix a small bug in iteration 1 implementation of Lane Offset Action. The current offset of actor's position was used to start the dynamic.

But the action actually starts with reference to the current centerline of existing path-control-unit. Hence the dynamic should always start from S=0 and the current existing offset of ego with path-control-units's reference centerline has to be added to the target offset specified by user.

gt-gen-simulator!111 (merged) An integration test added to above can help you understand the problem and fix.