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/Cargo.toml

@@ -30,6 +30,7 @@ bitflags = "1.2"
 rustix = "0.33.0"
 
 [target.'cfg(windows)'.dependencies]
+io-extras = "0.13.2"
 winapi = "0.3"
 
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