Provide consistant API endpoints for (skipped_)content_missing
These 2 endpoints are inconsistent (return type). In order to minimize the migration impact (and prevent the need for a synchronous upgrade of all SWH services at once), the idea is to add new (consistent) endpoints, probably using the result of swh-model#4641, and deprecate these 2 endpoints before getting rid of them in a (not so long) future.