Add a function to compute start and/or end of a way to write in a way:Layer

[BenOnTrack]

While processing ways, there should be a similar function as "LayerAsCentroid" that would write the start and/or end of the way in the layer (if possible configurable - nice to have). Be aware of ways that may not have start/end, such as "loop" ways (unless there would always be a start/end, based on how the way has been built on the pbf?).

That'd be great to have a similar function for relation, but you may face issue if the ways are not contiguous - unless we could have a check that returns nil if the relation is "discontinued, and that returns start and/or end if not.