Flexible configuration for the external notification module

[haxney]

With multiple different triggers and actions, the number of fields in the ExternalNotificationConfig was starting to grow large, and the interactions between the different fields was complicated.

Example

For example, if someone wanted the configuration

start the buzzer and vibra when receiving a direct message containing a bell character

then the ExternalNotificationConfig message would need a field:

bool alert_bell_direct_message_buzzer = 16;
bool alert_bell_direct_message_vibra = 17;

And so on for each combination of conditions and actions.

This commit

This commit introduces the ExternalNotificationCase message, which separates the condition (when to trigger an external notification) from the action (which pin to activate). The previous example would look like this:

case {
  condition {
    dest_direct: {}
    content_bell: {}
  }
  actions {
    output_buzzer: {}
  }
  actions {
    output_vibra: {}
  }
}

Adding new conditions (such as a message text containing a string prefix or being sent by a particular node) and actions (such as a fourth output pin) does not result in a combinatorial explosion of fields and keeps the implementation simpler.

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