feat(provider): updated the pubnub provider by updating fcm and apns payloads. (#214)

updated the pubnub provider by updating fcm and apns payload as per pubnub documentaton

https://www.pubnub.com/docs/general/resources/migration-guides/legacy-fcm-migration-guide

issue-211

Co-authored-by: prernagp <prernagp90@gmail.com>

Author: prernagp