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/src/file.rs

@@ -7,6 +7,16 @@ pub trait WasiFile: Send + Sync {
     fn as_any(&self) -> &dyn Any;
     async fn get_filetype(&mut self) -> Result<FileType, Error>;
 
+    #[cfg(unix)]
+    fn pollable(&self) -> Option<rustix::fd::BorrowedFd> {
+        None
+    }
+
+    #[cfg(windows)]
+    fn pollable(&self) -> Option<io_extras::os::windows::RawHandleOrSocket> {
+        None
+    }
+
     fn isatty(&mut self) -> bool {
         false
     }