Feature: CloudEvents component

Enabling CloudEvents component could make spin a great solution to serverless and event-driven applications. I am thinking about an event trigger that subscribes to a addressible broker, and whenever the broker emits an event, the event trigger will instantiate a wasm module to hanlde. Or the trigger itself can emits an event.

For example the trigger can subscribe to a knative event source.

I am proposing the following component sdk. Let me know your thoughts. @radu-matei

use anyhow::Result;
use spin_sdk::{
    event::{Event},
    event_component,
};

/// A simple Spin event component.
#[event_component]
fn trigger(event: Event) -> Result<()> {
    println!("{}", event);
    Ok(())
}

Start by reviewing the CloudEvents proposal, the Knative event source example, and the proposed Rust component SDK in the issue. Define the event trigger and broker behavior before identifying implementation and validation work; done means the scope and event-component interface are agreed.