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 T2849.
Depends on D4807.
Missing:
- [] cassandra implementation,
- [] inmemory implementations,
- [] feed kafka,
- [] consistent names,
- [] more tests,
- [] migration scripts,
- [] other stuff