Refactor swh.vault factory so migration tool can work with it

Currently, it's not possible to make the migration tool work with the vault due to how the VaultBackend (postgresql) constructor is implemented. The VaultBackend constructor expects the configuration required to instantiate its collaborator (VaultCache, ...). But the current migration tool (cli) does not pass those around, so the overall instantiation of the VaultBackend in that context just break [1] (last stacktrace).

• [1] #4305 (closed)

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Migrated from T4312 (view on Phabricator)