Option to save/read ExtImage as SpatRasterImage

[lambdamoses]

Say I rotated the image, resulting into ExtImage, and sometimes it can be quite large. The next time I don't want to read it back as ExtImage. Initially I thought I should get the same class I saved back, but this is not done in alabaster.matrix because all matrices are read as ReloadedMatrix which is based on DelayedArray. Would be cool to try something like TileDB which has sparse matrix support though, since subsetting HDF5Array is slow.