Avoid using shared_ptr for the linestring's kdtree

See https://github.com/maliput/maliput_sparse/pull/52#discussion_r1100765058 discussion

### Summary

https://github.com/maliput/maliput_sparse/blob/471c19ddd0913522813569d835dc5b569d25ab14/include/maliput_sparse/geometry/line_string.h#L274

- Use unique_ptr instead of shared_ptr
- This might trigger several changes across maliput_sparse maliput_osm as the linestring will stop being copyable.

To keep in mind(from comments):

> Then, it makes sense to have a view-type of the LineString that allows you to access it but not necessarily own the memory. It could be arranged somewhere else. For that to happen, we can change the builder to construct LineStrings at the Segment level, and then organize how we want them to be used. That means, we could derive LaneGeometries from the set of LineStrings or explicitly organize the pair of LineStrings at the moment of construction the LaneGeometry. This does not require a shared_ptr.