Add payloads to ExtIDs

Update the storage layer to support the model changes in swh-model!267 (closed)

I'm not entirely sure about the SQL schema migrartion. Running the SQL manually seems to work, but I couldn't get it to work from swh db upgrade. It seems to be an issue with my test environment rather than something with the upgrade SQL itself, though.

part of swh-model#2430

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Migrated from D8760 (view on Phabricator)