Implement function to encrypt relevant parts of iOS notification

Summary: This differential implements a function that encrypts parts of iOS notification payload that are important to user's privacy

Test Plan:

1. Pull cookie of currently logged in user from MariaDB.
2. Call this function in the keyserver code where iOS notification is constructed with hardcoded cookie id.
3. Send notification to the physical iOS device. Log content of send notification and ensure relevant fields are encrypted.

Reviewers: tomek, atul, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7813