Add softwareheritage-indexer backup

As a general rule of thumbs, it's good to have backup :) And we did up until we separated the db (T867).

Tables to backup:

• origin_intrinsic_metadata
• revision_intrinsic_metadata tables

The other ones may be of lesser importance:

• content_mimetype
• content_fossology_license

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Migrated from T880 (view on Phabricator)