[master] Treat Extra field in production request DB as JSON
As a result of changing the contents of the production request DB to store JSON blobs instead of pickle it becomes impossible for a member of the PPG to sign an old production request that contains pickled data as it gets automatically promoted to JSON causing the strings to no longer match. As the "Extra" field is now being updated it fails the checks at:
- https://gitlab.cern.ch/lhcb-dirac/LHCbDIRAC/-/blob/cb5d0275f0eed68d11aef13339e48295ae92f14a/LHCbDIRAC/ProductionManagementSystem/DB/ProductionRequestDB.py#L637
- https://gitlab.cern.ch/lhcb-dirac/LHCbDIRAC/-/blob/cb5d0275f0eed68d11aef13339e48295ae92f14a/LHCbDIRAC/ProductionManagementSystem/DB/ProductionRequestDB.py#L590
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*ProductionManagementSystem FIX: Treat "Extra" field as JSON to avoid unsignable requests
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