masking: Add method MaskingQuery.iter_masked_swhids
This will be used by swh-dataset to list all SWHIDs to mask before an export, instead of querying the database over and over while exporting.
Staging instance, all changes can be removed at any time
This will be used by swh-dataset to list all SWHIDs to mask before an export, instead of querying the database over and over while exporting.