feat(iroh-net)!: add subscribe method to Discovery trait (#2656)

## Description

Adds a `subscribe` method to the `Discovery` trait that returns an
`Option<BoxStream>`. The `subscribe` method will send `DiscoveryItems`
each time the discovery service discovers a remote node.

The magicsock is now subscribed to the discovery service and updates the
internal address book each time it receives a `DiscoveryItem`. The
source is marked as `Source::Discovery{ service: String }` and the
`Instant` that the magicsock received the information.

Users can now filter their list of `RemoteInfo`s for sources.

## Breaking Changes

- struct `RemoteInfo` now has field `sources`, which is a `Vec` of
`(iroh::net::endpoint::Source, Duration)`. The `Source` is how we heard
about the remote, and the `Duration` is how long ago we heard about it.
The `sources` field is ordered from the first time we heard about the
remote node to the most recent time we learned about the remote node, in
this session.

## 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.
- [x] Tests if relevant.
- [x] All breaking changes documented.

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Co-authored-by: Kasey Huizinga <ramfox@Kaseys-MBP.lan>
Co-authored-by: Divma <26765164+divagant-martian@users.noreply.github.com>
Co-authored-by: Floris Bruynooghe <flub@n0.computer>

Author: 4 people