[services][blob] Handle getting blob data

Summary:
Implemented basic logic for the Get RPC call.

C++ counterpart

Depends on D5700

Test Plan:

1. Create a blob using e.g. the C++ blob service (or in any other way)
2. Get the blob data with the Rust implementation
3. If the data is small, modify chunk_size to some small value to test if multiple parts are returned as expected

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5701