[report-service] Implement DB row item type

Summary:
This diff implements a struct that represents a row in the database. Also implemented conversions from/to attribute maps and other helper traits.

Depends on D8938

Test Plan: Tested later in the stack.

Reviewers: michal, patryk, varun, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8939