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

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.

iroh/src/discovery.rs

@@ -111,6 +111,7 @@
     doc = "[`LocalSwarmDiscovery`]: local_swarm_discovery::LocalSwarmDiscovery"
 )]
 
+use std::sync::Arc;
 use std::{collections::BTreeSet, net::SocketAddr, time::Duration};
 
 use anyhow::{anyhow, ensure, Result};
@@ -194,6 +195,8 @@ 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 {
@@ -445,6 +448,7 @@ mod tests {
 
     use iroh_base::SecretKey;
     use rand::Rng;
+    use testresult::TestResult;
     use tokio_util::task::AbortOnDropHandle;
 
     use super::*;
@@ -725,6 +729,21 @@ mod tests {
             .expect("time drift")
             .as_micros() as u64
     }
+
+    #[tokio::test]
+    async fn test_arc_discovery() -> TestResult {
+        let discovery = Arc::new(EmptyDiscovery);
+
+        let _ep = Endpoint::builder()
+            .add_discovery({
+                let discovery = discovery.clone();
+                move |_| Some(discovery)
+            })
+            .bind()
+            .await?;
+
+        Ok(())
+    }
 }
 
 /// This module contains end-to-end tests for DNS node discovery.