App Notifications through SDNA triggers 1 - base implementation

[lucksus]

First half of: #363

• spec out all the needed functions / interfaces
• add new functions to Ad4mClient
• types/handler boilerplate
• implement notification trigger in ad4m executor

This adds the core functionality for ADAM Notifications, i.e. apps registering notifications, executor sending a signal, waiting for the user to confirm the notification install request and then check the notification prolog query on every perspective change - and eventually send out the notification through a subscription.

Whats missing for a complete implementation:

• notification webhook push
• UI in ADAM Launcher to deal with notification install requests