[chore]: Add a way to reload ClusterManager with map rect

[noahlit]

Currently if using SwiftUI map, we need to pass MKCoordinateRegion into the ClusterManager when reloading the clusters/annotations. However, I found this causes all clusters and map annotations to disappear when zoomed all the way out.

This PR adds a function to ClusterManager so we can reload with an existing MKMapRect instance rather than having the library convert MKMapCoordinateRegion into MKMapRect.

Old (From README):

Map()
.onMapCameraChange(frequency: .onEnd) { context in
  Task.detached { await clusterManager.reload(mapViewSize: mapSize, coordinateRegion: context.region) }
}

New:

Map()
.onMapCameraChange(frequency: .onEnd) { context in
  Task.detached { await clusterManager.reload(mapViewSize: mapSize, mapRect: context.rect) }
}

Fixes: #5