[blob-service] Implement db type conversions

Summary:
Adds trait implementation to the previously added interfaces (see parent diff for details).

Depends on D8442, D8438

Test Plan: Tested later in the stack

Reviewers: michal, jon, varun, patryk

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8443