Push notifications handling guide

[neekolas]

We need to provide some resources for devs around push notification handling. We also need to expose methods for actually taking in the push notifications when they are received by the notification extension.

Topics

Welcome topics have the format:

`/xmtp/mls/1/w-${hexEncode(installationId)}/proto`

Group message topics have the format:

`/xmtp/mls/1/g-${hexEncode(groupId)}/proto`

Clients should listen to these topics and then decode the payloads.

Payload types

• Welcome messages are protobuf messages encoded with mlsv1.WelcomeMessage
• Group messages are protobuf messages encoded with mlsv1.GroupMessage

What to do with messages

• Once payloads have been decoded from the push notification callers can pass them to process_streamed_welcome_message and process_streamed_group_message in our SDKs (these functions are not exposed outside of Rust).

Tasks

Beta Give feedback

1. Expose FFI interface for processing welcome and group messages to libxmtp
2. Add ability for callers to pass in Welcome messages to iOS SDK for processing
3. Add ability for callers to pass in Group messages to iOS SDK for processing
4. Add ability for callers to pass in Welcome messages to Android SDK for processing
5. Add ability for callers to pass in Group messages to Android SDK for processing
6. Add ability in RN

[nplasterer]

Would be nice to have a example running in RN for this but going to push this down in priority in favor of the HMAC work for now. PR started here: xmtp/xmtp-react-native#347