Support an incident.io notifier

[rorymalcolm]

Creating an issue here, following a standard from an earlier issue (#3508).

I would like to build an incident.io notifier - adding direct support for sending alerts from Alertmanager to incident.

This would allow for Alertmanager to have different retry behaviour than its standard webhook behaviour in the case of rate limits, alongside a number of other improvements to the usability.

Creating this issue to understand what's required to land a proposal for a new notifier like this - which I'd be happy to implement.

[grobinson-grafana]

Hi! 👋

We are happy to accept new integrations that are in-demand by the community. I can see that there is a lot of support for this, so I would be happy to review it and get it merged. Take one of the existing integrations as a reference (perhaps Pagerduty as I think it is the most similar to incident.io) and go from there, everything else should be pretty straightforward.

[rorymalcolm]

Hey @grobinson-grafana , thanks for getting back promptly - as above, have started work in that PR!