fix(wasi): enable all WasiFiles to be pollable (#3913)

Currently, the use of the downcast method means that you have to use one
of the hard-coded types. But Enarx needs to define its own `WasiFile`
implementations. This works fine, except the resulting files cannot be
used in poll because they aren't part of the hard-coded list.

Replace this with an accessor method for the pollable type in
`WasiFile`. Because we provide a default implementation of the method
and manually implement it on all the hard-coded types, this is backwards
compatible.

Signed-off-by: Nathaniel McCallum <nathaniel@profian.com>

crates/wasi-common/cap-std-sync/src/file.rs

@@ -26,6 +26,15 @@ impl WasiFile for File {
     fn as_any(&self) -> &dyn Any {
         self
     }
+    #[cfg(unix)]
+    fn pollable(&self) -> Option<rustix::fd::BorrowedFd> {
+        Some(self.0.as_fd())
+    }
+
+    #[cfg(windows)]
+    fn pollable(&self) -> Option<io_extras::os::windows::RawHandleOrSocket> {
+        Some(self.0.as_raw_handle_or_socket())
+    }
     async fn datasync(&mut self) -> Result<(), Error> {
         self.0.sync_data()?;
         Ok(())