model: Add payload to ExtID class

Open Timothy Sample requested to merge generated-differential-D8759-source into master

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 where Stefano suggests using a generic payload mechanism for ExtIDs and where we decided on using object storage for the payload.

Migrated from D8759 (view on Phabricator)

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