Revert "postgresql: drop fallback for extra_headers in Revision.metadata"

This reverts commit 8435607b

There are still unmigrated extra_headers in Revision.metadata. For example, swh:1:rev:04a3d20ff411409ab98474892b2bb1713bde0f7f

(this re-opens swh/infra/sysadm-environment#2564)

approved this merge request

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merged

changed the description