[blob-service][HTTP] Get blobs by blob_hash

Summary:
Resolves ENG-3967.
Changed Blob Service HTTP get endpoint to accept blob_hash instead of holder.

Test Plan: Uploaded an image to blob service with blob_hash = test_hash1, then I wasa able to download it by entering http://BLOB_SERVICE/blob/test_hash1 in the browser.

Reviewers: jon, varun, michal

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8206