Add optional checksum retrieval to IrodsAPI.get_objects()

[mikkonie]

It would be very useful for e.g. ProjectIrodsFileListAPIView to also display the checksum for each data object.

For the sake of completeness, I believe this should be returned as a list of replica/checksum pairs, as the same file may exist in multiple replicas. Alternatively, maybe the user could supply a specific resource name..

This could be done together with #1996 and #1997 as they are somewhat related.