Add support for ExtID in the storage

This endpoint allows to query the storage for known external identifier (typ. revisions from their original VCS type intrinsic identifier.)

The underlying data structure is filled via the extid_add() endpoint (which will be the responsibility of loaders).

Related to #2849 (closed).

Depends on swh-model!148 (closed).

Missing:

• cassandra implementation,
• inmemory implementations,
• feed kafka,
• consistent names,
• more tests,
• migration scripts,
• other stuff

---

Migrated from D4698 (view on Phabricator)