feat(iroh)!: expose DynProtocolHandler (#3366)

## Description

This makes `DynProtocolHandler` public to make building custom routers
easier.

## Breaking Changes

* `iroh::protocol::RouterBuilder::accept` now takes `impl Into<Box<dyn
DynProtocolHandler>>` instead of `impl ProtocolHandler`. Because of a
blanket `From` impl this change does not need any changes by users: you
can still pass any `impl ProtocolHandler` to `accept`. Additionally, if
you have your own builder struct upstream, you can now also pass a
`Box<dyn DynProtocolHandler>` to `accept`, which wasn't possible
previously.

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
-->

## Change checklist
<!-- Remove any that are not relevant. -->
- [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] All breaking changes documented.
- [ ] List all breaking changes in the above "Breaking Changes" section.
- [ ] Open an issue or PR on any number0 repos that are affected by this
breaking change. Give guidance on how the updates should be handled or
do the actual updates themselves. The major ones are:
    - [ ] [`quic-rpc`](https://github.com/n0-computer/quic-rpc)
    - [ ] [`iroh-gossip`](https://github.com/n0-computer/iroh-gossip)
    - [ ] [`iroh-blobs`](https://github.com/n0-computer/iroh-blobs)
    - [ ] [`dumbpipe`](https://github.com/n0-computer/dumbpipe)
    - [ ] [`sendme`](https://github.com/n0-computer/sendme)

Author: Frando

iroh/src/protocol.rs

@@ -217,11 +217,21 @@ impl<T: ProtocolHandler> ProtocolHandler for Box<T> {
     }
 }
 
+impl<T: ProtocolHandler> From<T> for Box<dyn DynProtocolHandler> {
+    fn from(value: T) -> Self {
+        Box::new(value)
+    }
+}
+
 /// A dyn-compatible version of [`ProtocolHandler`] that returns boxed futures.
 ///
-/// We are not using [`n0_future::boxed::BoxFuture] because we don't need a `'static` bound
-/// on these futures.
-pub(crate) trait DynProtocolHandler: Send + Sync + std::fmt::Debug + 'static {
+/// Any type that implements [`ProtocolHandler`] automatically also implements [`DynProtocolHandler`].
+/// There is a also [`From`] impl to turn any type that implements [`ProtocolHandler`] into a
+/// `Box<dyn DynProtocolHandler>`.
+//
+// We are not using [`n0_future::boxed::BoxFuture] because we don't need a `'static` bound
+// on these futures.
+pub trait DynProtocolHandler: Send + Sync + std::fmt::Debug + 'static {
     /// See [`ProtocolHandler::on_connecting`].
     fn on_connecting(
         &self,
@@ -276,8 +286,7 @@ impl ProtocolMap {
     }
 
     /// Inserts a protocol handler.
-    pub(crate) fn insert(&mut self, alpn: Vec<u8>, handler: impl ProtocolHandler) {
-        let handler = Box::new(handler);
+    pub(crate) fn insert(&mut self, alpn: Vec<u8>, handler: Box<dyn DynProtocolHandler>) {
         self.0.insert(alpn, handler);
     }
 
@@ -348,8 +357,18 @@ impl RouterBuilder {
 
     /// Configures the router to accept the [`ProtocolHandler`] when receiving a connection
     /// with this `alpn`.
-    pub fn accept(mut self, alpn: impl AsRef<[u8]>, handler: impl ProtocolHandler) -> Self {
-        self.protocols.insert(alpn.as_ref().to_vec(), handler);
+    ///
+    /// `handler` can either be a type that implements [`ProtocolHandler`] or a
+    /// [`Box<dyn DynProtocolHandler>`].
+    ///
+    /// [`Box<dyn DynProtocolHandler>`]: DynProtocolHandler
+    pub fn accept(
+        mut self,
+        alpn: impl AsRef<[u8]>,
+        handler: impl Into<Box<dyn DynProtocolHandler>>,
+    ) -> Self {
+        self.protocols
+            .insert(alpn.as_ref().to_vec(), handler.into());
         self
     }