[reports-service] Introduce database client

Summary:
Similiar to other services, created a struct representing DynamoDB client.
Created basic interfaces to store and retrieve reports. They're very simple but sufficient for now.

Depends on D8939, D8895

Test Plan: Storing reports was tested in D8895, other methods are tested together with HTTP interface (later in the stack)

Reviewers: michal, varun, patryk, jon

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8942