[services][backup] Add database client module

Summary:
Introduced DynamoDB database client - the same way as it is implemented in Blob and Identity services.

The Error type is properly implemented two diffs later.

Depends on D6168

Test Plan: Build does not fail.

Reviewers: varun, tomek, jon, max

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6177