[services][blob] Extract BlobItem creation

Summary:
This one was actually a starter task but I'll need this one inside HTTP server too so doing it now.

Depends on D7441

Test Plan: Blob service builds and runs. Ran integration tests to ensure items are still created correctly.

Reviewers: marcin, michal, jon, varun

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7442