[keyserver] convert existing logic for building userPushInfoPromises to an async function called generateNotifUserInfoPromise

Summary:
The logic for building userPushInfoPromise will be really simlar when we build userRescindInfoPromise so I built generateNotifUserInfoPromise to reuse this logic in a subseqent diff

Depends on D6307
Linear Task: ENG-2644

Test Plan:
flow and please see video showing no regressions with the notifications.

Reviewers: ashoat, tomek, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6829