feat(iroh): implement Discovery for Arc'ed Discovery types (#3107)

## Description

Nothing in the trait stops it from working for discovery structs which
are Arc'ed.  Not all discovery types are in control of the users so
this is helpful.

I stumbled upon this because StaticProvider is not Clone.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

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

iroh/src/discovery.rs

@@ -111,7 +111,7 @@
     doc = "[`LocalSwarmDiscovery`]: local_swarm_discovery::LocalSwarmDiscovery"
 )]
 
-use std::{collections::BTreeSet, net::SocketAddr, time::Duration};
+use std::{collections::BTreeSet, net::SocketAddr, sync::Arc, time::Duration};
 
 use anyhow::{anyhow, ensure, Result};
 use futures_lite::stream::{Boxed as BoxStream, StreamExt};
@@ -194,6 +194,8 @@ pub trait Discovery: std::fmt::Debug + Send + Sync {
     }
 }
 
+impl<T: Discovery> Discovery for Arc<T> {}
+
 /// The results returned from [`Discovery::resolve`].
 #[derive(Debug, Clone)]
 pub struct DiscoveryItem {