vault/api_views: Enable git bare cooking for all supported objects
It enables to cook a git repository for all supported SWH object types: directory, release, revision and snapshot.
Previously only a revision could be cooked as a git repository through the Web API.
This superseeds !1182 (closed) by updating endpoint documentation and fixing tests.
To be noted, while testing this I found a small bug in the vault related to the cooking of a release as a git barre bundle. I pushed a fix in swh-vault!190 (merged).
Edited by Antoine Lambert