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