[services-lib] Introduce Blob Client

Summary:
Part of ENG-4597.

This diff introduces a foundation for blob client in comm-services-lib. The client is hidden behind a crate feature so services that do not need it do not have to depend on it (e.g. feature-flags).

Test Plan: Cargo build in comm-services-lib, blob and feature-flags.

Reviewers: michal, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8777