[keyserver] Populate public_key column of cookies table on register

Summary:
Populate the public_key column of the cookies table with primaryIdentityPublicKey included in register request on successful registration.

Depends on D6735

Test Plan:
public_key column populated as expected following successful registration on native (it's not possible to register a "traditional" account on web at this time):

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6736