[services][backup] Scaffold blob client

Summary:
Added initial structure for blob client module, with submodules for get and put clients.
Scaffolded both client structs, usage of which is similiar to this, in both cases:

let client = GetClient::start(holder).await;

while let Some(data) = client.get().await { ... }
// or put() for PutClient

let result = client.terminate().await;

This was somehow inspired on C++ usage, which was like that:

blob_client_initialize_cxx()
blob_client_write_cxx()
blob_client_blocking_read_cxx()
blob_client_terminate_cxx()

Added basic comments to their methods, more detailed description and implementation is going to be added in subsequent diffs.

Depends on D6161

Test Plan: Service builds successfully. Added code does nothing yet.

Reviewers: varun, tomek, jon, max

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6165