Send alerts with Apprise

Apprise is a python lib who can send message to multiple services (Matrix, Slack…).

We can add a notify_with_apprise in argos/server/alerting.py to use apprise to send alerts.

Config file example:

apprise:
  - name: matrix
    uri: matrixs://user:pass@hostname:port/!room_id
  - name: slack
    uri: slack://botname@TokenA/TokenB/TokenC/Channel

I can do the implementation if we are agree with this specifications