Introduce length() method in apn.Notification flow-typed

Summary: This diff extends apn.Notification type with length method provided by untyped version of node-apn library. We will need in furhter diffs

Test Plan: Temporarily place call to langth() of apn.Notification instance and ensure flow type checker does not complain.

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4617