feat(server): give Server::local_addr a more general type

Allows `local_addr` to work for any executor type, rather than just the default `Exec`. The
underlying `SpawnAll::local_addr()` is already similarly general, so no other changes are needed
other than adding the extra type parameter to the `impl`.

src/server/mod.rs

@@ -134,7 +134,7 @@ impl Server<AddrIncoming, ()> {
 }
 
 #[cfg(feature = "tcp")]
-impl<S> Server<AddrIncoming, S> {
+impl<S, E> Server<AddrIncoming, S, E> {
     /// Returns the local address that this server is bound to.
     pub fn local_addr(&self) -> SocketAddr {
         self.spawn_all.local_addr()