Emit Download and Append events

[Frando]

Choose one: a 🙋 feature

This PR adds a subscribe method to the Feed which returns a Receiver<Event>. Currently, there's two events being emitted: Download and Append. More can be added over time/when needed.

I think this will be important for upcoming replication integration.

Checklist

• [x] tests pass
• [x] tests and/or benchmarks are included

Context

Semver Changes

minor as it's only an API addition I think