[report-service] Add report types structs

Summary:
Introduce structs for report types, to be used in the report service.
They're somehow counterparts of lib/types/report_types.js but modified to be properly serialized/deserialized by Serde.

Depends on D8865

Test Plan: Tested together with further diffs.

Reviewers: michal, patryk, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8937