Implement webhook endpoint for garo-server

[marcofranssen]

Garo server needs a /webhook endpoint which can be consumed by github.com

The webhook endpoint should be protected by a configurable secret to prevent abuse of this API.

IP whitelisting can be arranged via a LB / security group

The check_run webhook should enable us to check if a build failed due to no available runner. If this occurs Garo-server needs to track this so the agents can retrieve for which repositories a runner has to be deployed and as well which runs need to be retriggered.

[marcofranssen]

@npalm could you add the involved curl requests to retrieve the run-failure?

[npalm]

I would suggest we implement a webhook and dispatch event to a command handler for specific events. to start with we only need an event listener for the check_run event.