Add sha512 as a valid field in dsc metadata
It has existed, at least at some point in the past, even though I'm currently unable to reproduce a dsc with that field in it. (Hence the lack of test fixture...)
Related to swh/infra/sysadm-environment#3143 (closed).
Test Plan
This was necessary to run the migration script in swh/infra/sysadm-environment#3143 (closed) to completion.
Migrated from D5430 (view on Phabricator)