[keyserver] Populate public_key column of cookies table on login

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

Test Plan:
public_key column populated as expected following successful INTERACTIVE login on both native and web:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6735