Add payloads to ExtIDs
This is a follow up to !849 (closed) , which I cannot update due to the Phabricator migration.
I've rebased the changes on master. @vlorentz updated the Cassandra keys in the meantime, so the rebase addressed that issue. I've updated the Postgres column comments as requested.
part of swh-model#2430
Edited by vlorentz