Add a generic Dart library for notifications

[nickelskevin]

Is your feature request related to a problem? Please describe.
As described in this document, there is an opportunity to provide an end-to-end encrypted, peer-to-peer, surveillance free notification service.

Describe the solution you'd like
The approach needs to be discussed in the architecture call.
Adding to the backlog so we can plan accordingly.

Describe alternatives you've considered
This could be accomplished as a widget only, but a library for sending notifications for IoT or other applications may also makes sense.

[gkc]

@nickelskevin Now that notify and monitor have been implemented, do we want to revisit this and think about making a stand-alone library that developers can include just for this purpose?

[gkc]

@gkc start thinking about this again

[ksanty]

@gkc should we re-prioritize to high for PR43 sprint planning?

[gkc]

I've flagged it for architecture discussion. The question to be answered is whether we want a standalone library just for sending / receiving notifications.