[blob-service] Introduce database types

Summary:
Part of ENG-4269.

Introduced types and interfaces to represent items in the database, according to the new schema (see linear task).
Implementations added in the next diff.

Depends on D8441, D8439.

Test Plan: Tested in the next diff

Reviewers: michal, jon, varun, patryk

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8442