Implement a broadcast/subscription interface to clock::Clock

[ralexstokes]

It could be nice to have a way to broadcast clock events (new slot, new epoch) similar to how we stream slots.

I have this example handy so I'll link here: https://github.com/paradigmxyz/reth/blob/main/crates/payload/builder/src/events.rs

it would kind of look like this; just for the clock events, rather than payload events