Collect extrinsic metadata

This is a meta-task that tracks activities related to //collecting, storing, indexing and showing (API and WebApp)// metadata coming from external sources.

We want a working pipeline (see #1344 (closed) and #1739 (closed) about specs), tested on at least a real-world instance, e.g.:

• extrinsic origin metadata from GitHub #1740 (closed)
• extrinsic metadata from SwMath #2031
• ClearlyDefined metadata

Metadata workflow boards can be found at (do we need two separate workboards?):

• https://forge.softwareheritage.org/project/board/73/
• https://forge.softwareheritage.org/project/board/149/

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Migrated from T2202 (view on Phabricator)