As an app developer I want to invoke my Trigger using `fn call` so that I don't have to use `curl`

[rikgibson]

With the introduction of Triggers, and the ability for a Function to exist independently of any Triggers, the behaviour of fn call will change to invoke a function directly, e.g fn call my-app my-func. This however leaves the CLI unable to invoke a function via a Trigger, meaning that a user would need to use curl or similar to invoke a Trigger HTTP endpoint.

One proposal here would be to extend the behaviour of fn call to support an optional --trigger option, for example:

fn call my-app my-func --trigger my-trigger

Instead of invoking the direct invocation endpoint for the function, this would instead retrieve the Trigger endpoint from the server and make an HTTP request to this endpoint instead.