[blob-service] Implement download logic with new db

Summary:
Part of ENG-4269.

Most of this code is copy-pasted from http::handlers::blob::get_blob_handler

Decided to create a separate download object, similar to "upload sessions" we already have. The reasoning for this is that we typically have the following workflow:

1. Get the download object metadata (size etc.)
2. Set download params (range, format, streaming options)
3. Stream the data to the client

Depends on D8453.

Test Plan: Tested later in the stack

Reviewers: michal, varun, jon, patryk

Reviewed By: michal, patryk

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8454