Port SBET Reader and Writer

This changes `PDAL_ADD_DRIVER` slightly. Instead of propagating `PDAL_TARGET_OBJECTS` to `PARENT_SCOPE` within the macro, we pass them back to the driver CMakeLists, where they can be combined with other object libraries and propagated to `PARENT_SCOPE`. This is necessary for drivers that may have both a reader and a writer (e.g., SBET).