Limit function triggers to only one type?

ATM a function is triggered both by HTTP and Knative Events, eg. Azure Functions allows only one trigger for a [function](https://docs.microsoft.com/en-us/azure/azure-functions/functions-triggers-bindings).

Do we want to follow Azure Fn and limit triggers? We will have to find a way how to disable HTTP triggers, which are provided by Knative Serving.