[services][blob] Introduce database client

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

The interface of the C++ implementation is here.

Depends on D5680

Test Plan: Only ensured that build does not fail.

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5681