Event discovery

**Problem**
Event discovery have been a needed feature with many attempts to implement it. There's a need that users can discover various existing eventing components. One attempt to solve it was via the event registry and EventType CRD #929. The new [Discovery API](https://github.com/knative-sandbox/discovery) offers a more promising approach.

We need a more unified story for discovery in eventing that potentially leverages the Discovery API and aligns the event registry and EventType features accordingly.

**Additional context**

- [Current Discovery API design doc.](https://github.com/knative-sandbox/discovery/blob/master/docs/proposal.md)
- [Initial Discovery API design doc.](https://docs.google.com/document/d/1CVMI0IpGqZH7j64-KQ6_8hJVUkeYEM0pdXslJ10a69E/edit#)

Start by reading the current Discovery API design document and the initial design document linked in the issue. Compare their approach with the existing event registry and EventType CRD work referenced in the issue. Done means there is an agreed unified discovery story for eventing and a clear alignment plan for those existing features.

Written by the indexing model from the issue text.