[services][blob] Implement Get blob HTTP handler

Summary:
Resolves ENG-3522

Added implementation of the GET /blob/{holder} endpoint. The implementation is very similar to the Get blob RPC endpoint.

Depends on D7464

Test Plan:
Created a blob larger than 5MB through gRPC API and called this endpoint using Postman. Ensured the response is correct.
Checked if HTTP 404 is returned when blob doesn't exist.

Reviewers: marcin, tomek, michal, jon

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7465