This revision adds payload and payload type fields to
the ExtID class. The intent is to use these fields to store
Disarchive specifications to support recovering source tarballs.
See https://sympa.inria.fr/sympa/arc/swh-devel/2022-02/msg00022.html
where Stefano suggests using a generic payload mechanism for ExtIDs
and https://sympa.inria.fr/sympa/arc/swh-devel/2022-05/msg00027.html
where we decided on using object storage for the payload.