Split by square should have a config toggle to use OSM buildings or not

[spwoodcock]

• Currently when using split by square, we download a data extract of the area, then omit any squares that contain no buildings - great for projects where all the digitisation is complete & buildings known!
• But if we need to map new buildings anywhere, this means we have no tasks to assign the new buildings to.
• We should make this an advanced config 'opt-in' option, as it's mostly for advanced users.
• The default should just be a complete set of squares, with nothing omitted.

[linear]

TECH-1091