feat(iroh): Gossip client (#2258)

## Description

This makes gossip available in iroh. Using iroh-gossip directly, while
not horrible, is a bit verbose.

## Breaking Changes

~~Not sure. It exports a few more things, and adds things. But in theory
it should not modify existing things.~~

There should be none, and the semver test seems to agree...

## Notes & open questions

- ~~There can be some scenarios where this can cause trouble. E.g. when
subscribing and then unsubscribing to a topic that is also used for doc
sync.~~

This ^ is taken care of, since docs no longer use subscribe_all!

How to deal with missed messages due to slow reader:

I think that relying on guaranteed delivery for gossip is not a good
idea in any case. so maybe just sending a Lagged but then continuing is
best? Forcing the client to resubscribe could be a bit annoying.

## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.