[keyserver] Refactor registerOrLogIn to take callbacks

Summary:
issue: ENG-7267
I will need to use a different function in place of fetchCallUpdateOlmAccount, so I am refactoring registerOrLogIn to take callbacks

Test Plan: Ran my nix script that creates a new keyserver, then ran yarn dev and checked that the admin was correctly registered with identity

Reviewers: marcin, kamil

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11540