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>

Cargo.lock

@@ -3204,6 +3204,7 @@ dependencies = [
  "bitflags",
  "cap-rand",
  "cap-std",
+ "io-extras",
  "rustix",
  "thiserror",
  "tracing",