feat(iroh): re-export n0_watcher::Watcher trait (#3356)

## Description

Re-export the `n0_watcher::Watcher` so it is easier for our users to use
heavily relied on methods like `Endpoint::node_addr`.

This becomes especially painful when you are working on any protocols
using iroh.

## Notes & open questions

Should this be exported from a different module?
`iroh::endpoint::Watcher`?


## Change checklist
- [x] Self-review.

---------

Co-authored-by: “ramfox” <“kasey@n0.computer”>

Author: ramfox

iroh/src/lib.rs

@@ -275,6 +275,7 @@ pub use iroh_base::{
     KeyParsingError, NodeAddr, NodeId, PublicKey, RelayUrl, RelayUrlParseError, SecretKey,
 };
 pub use iroh_relay::{http::Protocol as RelayProtocol, node_info, RelayMap, RelayNode};
+pub use n0_watcher::Watcher;
 
 #[cfg(any(test, feature = "test-utils"))]
 pub mod test_utils;