Allow grouping Push Notifications

[ArionMiles]

I don't know if this is a good place to file this issue, since it mostly pertains to AWS Pinpoint and not Amplify in general.

Is your feature request related to a problem? Please describe.
Every notification sent through Pinpoint becomes an individual notification. For a usecase like a chat application, it's preferred if notifications coming from the same "group" were grouped together in the notifications shade. This is being done in many popular chat apps like WhatsApp and Telegram. This is only android specific though, I don't know if Apple devices group notifications.

Firebase Cloud Messaging allows adding a tag property while sending a notification so that the notifications are grouped together. See here

Describe the solution you'd like
Allow specifying a "tag" property like in Firebase Cloud Messaging for AWS Pinpoint methods.

Apologies if this isn't the right place to file this issue, let me know where I can raise issues regarding AWS Pinpoint.