Support simplified camera set IO

[russelldj]

Currently, we can only read camera sets in from metashape and not export them in any format. This is problematic because we frequently need to subset cameras to a geospatial region, which can be time consuming if it requires checking exif metadata. It would be useful to be able to save out the subset of camera files to avoid this repeated computation. A simple json should be sufficient, capturing things like the folder path to the images, the camera models, and then for each camera, the filename, rotation, translation, and scale, and which sensor it corresponds to. Optionally, the GPS data could be included.