Offline maps tile region boundaries

Nobody has claimed this yet.

## New Feature

I'd like to be able to highlight the geographic region accounted for by any offline TileRegion in the TileStore. Using the method below, I can retrieve the center coordinate of the offline tile region, but I don't see any way to grab the northwest and southeast corners of the region boundary.

```
TileStore.default.allTileRegions { result in
switch result {
case let .success(tileRegions):
print(tileRegions)
for region in tileRegions {
TileStore.default.tileRegionGeometry(forId: region.id, completion: { result in
switch result {
case let .success(tileRegionGeometry):
print(tileRegionGeometry) //center point of region
default:
break
}
})
}
default:
break
}
}
```

## Why
As far as i can tell, I can retrieve the center point of a region in the TileStore, but not its boundary. The boundary would be useful as I could then let the user know whether they've already download the tiles for a given area.

Start with TileStore.default.allTileRegions and tileRegionGeometry(forId:), which the issue identifies as the current offline-region entry points. Trace what geometry is returned and how TileRegion data is exposed; the work is done when callers can obtain each region's boundary, including its northwest and southeast corners, to compare it with a requested area.

Written by the indexing model from the issue text.