[keyserver] Split uploading blob and assigning holders

Summary:
For some callers it might be desirable to choose the ordering of these operations and perform them sequentially.

Depends on D9940

Test Plan: Check if calling the new function has the same result as the old one.

Reviewers: kamil, marcin

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9941