github-gateway: Add event type to event parameters

[discordianfish]

I want to run workflows/steps conditionally on the event type of a webhook.

The github-gateway adds a webhook for all events and passes the incoming webhook body as-is to the sensor. Unfortunately the github api doesn't return the type of the webhook in the body but only as http header: https://developer.github.com/webhooks/#delivery-headers
This means there is no well known name to distinguish between events.

We should add a event-type field to the message. The only way I see is to unmarshal the body in the gateway, add a new field and unmarshal it again. Is there a better way?