BoxedTransport: Add methods to downcast to a specific transport type

These functions are useful for getting access to an application-specific
custom transport type that has additional methods.

http/src/transport/boxed_transport.rs

@@ -14,6 +14,8 @@ use trillium_macros::{AsyncRead, AsyncWrite};
 pub(crate) trait AnyTransport: Transport + Any {
     fn as_box_any(self: Box<Self>) -> Box<dyn Any>;
     fn as_box_transport(self: Box<Self>) -> Box<dyn Transport>;
+    fn as_any(&self) -> &dyn Any;
+    fn as_mut_any(&mut self) -> &mut dyn Any;
     fn as_transport(&self) -> &dyn Transport;
 }
 impl<T: Transport + Any> AnyTransport for T {
@@ -23,6 +25,12 @@ impl<T: Transport + Any> AnyTransport for T {
     fn as_box_transport(self: Box<Self>) -> Box<dyn Transport> {
         self
     }
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn as_mut_any(&mut self) -> &mut dyn Any {
+        self
+    }
     fn as_transport(&self) -> &dyn Transport {
         self
     }
@@ -85,6 +93,24 @@ impl BoxedTransport {
     pub fn downcast<T: 'static>(self) -> Option<Box<T>> {
         self.0.as_box_any().downcast().ok()
     }
+
+    /**
+    Attempt to get a reference to the trait object as a specific transport type T. This will only
+    succeed if T is the type that was originally passed to [`BoxedTransport::new`], and will return
+    None otherwise
+    */
+    pub fn downcast_ref<T: Transport>(&self) -> Option<&T> {
+        self.0.as_any().downcast_ref()
+    }
+
+    /**
+    Attempt to get a mutable reference to the trait object as a specific transport type T. This
+    will only succeed if T is the type that was originally passed to [`BoxedTransport::new`], and
+    will return None otherwise
+    */
+    pub fn downcast_mut<T: Transport>(&mut self) -> Option<&mut T> {
+        self.0.as_mut_any().downcast_mut()
+    }
 }
 
 impl Deref for BoxedTransport {