[blob-service] Introduce database client

Summary:
Part of ENG-4269.

Introduced a new DatabaseClient class to interface DDB.

Depends on D8443.

Test Plan:
Tested together with 2 following diffs

Created a custom testing sandbox to play with these:

1. Checkout this stack (this + 2 next diffs)
2. Apply this patch and copy the tests.rs file contents attached there to the services/blob/src/database/client/tests.rs.
3. cargo run

Reviewers: michal, varun, jon, patryk

Reviewed By: michal, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8450