[report-service] Add report types DB conversions

Summary:
Add DynamoDB conversion for report types introduced in previous diff.
Also introduced DDB constants. These will be used in subsequent diffs.

Depends on D8937

Test Plan: Unit tests, also tested together with further diffs.

Reviewers: michal, patryk, varun, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8938