[wip] refactor(iroh-gossip)!: dispatch gossip events and updates by t…

…opic (#2570)

## Description

This PR changes the main public `iroh_gossip` to keep track of
client-side gossip subscriptions. The `net::Gossip` struct now keeps
track of client-side subscribers per topic, which are made up of a pair
of two streams/channels: from the client to the actor a stream of
updates (outgoing messages) and from the actor to the client a stream of
events (incoming messages). Once all client streams&sinks for a topic
are dropped, the topic is being quit.

This builds on the client API added in #2258, but completely removes the
`dispatcher` module, integrating its features directly into the gossip
actor. See below for a short list of the API changes. The new API can be
browsed
[here](https://n0-computer.github.io/iroh/pr/2570/docs/iroh/gossip/net/index.html).

The refactor turned out bigger than initially intended, sorry for that,
but I did not see a good way to reduce the scope.

What's still missing (can also be follow-ups)?:

- [ ] Review the new public API
- [ ] Align the client API to the iroh_gossip API. The `GossipTopic` can
be made to work on both the client and the native API, as it only deals
with streams and sinks.

## Breaking Changes

* `iroh_gossip::dispatcher` is removed with everything that was in it.
use the new API from `iroh_gossip::net::Gossip` instead (see below).
* `iroh_gossip::net::Gossip` methods changed:
  * changed: `join` now returns a `GossipTopic`
* removed: `broadcast`, `broadcast_neighbors`, `subscribe`,
`subscribe_all`, `quit`.
* for `subscribe` use `join` instead, which returns a `GossipTopic`
* for `broadcast` and `broadcast_neighbors` use the respective methods
on `GossipTopic` .
* `quit` is obsolete now, the topic will be quitted once all
`GossipTopic` handles are dropped.
      * `subscribe_all` is no longer available
* `iroh_gossip::net::JoinTopicFut` is removed (is now obsolete)

## Notes & open questions

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## Change checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.