[blob-service] Call service methods from handlers

Summary:
Part of ENG-4269.

Replaced HTTP handlers logic with calls to the BlobService class

Depends on D8457.

Test Plan:
Blob service integration and performance tests pass at this point.
Played with Postman to manually test the endpoints - both successful and failure cases (blob/holder not found, already exists, empty upload, invalid inputs)

Reviewers: michal, varun, jon, patryk

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8458