[services][blob] Add assign holder HTTP endpoint

Summary:
This diff implements endpoint to assign holder to a blob instance. Details are in the linear task. Ensuring that the blob exists is done in a separate diff.

Depends on D7466

Test Plan:
Executed this endpoint for:

• non-existing blob - false is returned in response
• existing blob - true is returned in response. Also checked if holder is assigned to blob in DB (a reverse index is created).
• already existing holder - HTTP 409 is returned

Reviewers: marcin, tomek, michal, jon

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7482