c#: use a native c file for exports that only call out to native calls

[jsturtevant]

          > > Could you suggest another approach?

WASI exports require WASI SDK and c-lang compilation anyway. We could also generate C code instead, just for this particular export type.

And then <NativeFileReference Include="wasi_exports.c" /> in csproj

Originally posted by @pavelsavara in https://github.com/bytecodealliance/wit-bindgen/issues/1145#issuecomment-2646399738