Feture request: Add `--async` flag support for C++

[MendyBerger]

wit-bindgen c and rust have --async to lift/lower functions as async; cpp has none, it only sync-lifts.

This makes C++ components unusable on wasip3, where some things like wasi:filesystem/stdio are async-only: any blocking I/O traps with cannot block a synchronous task before returning, and there's no generator-level way to make the export async.

Would love to have an --async <FILTER> in the cpp generator with the same semantics as c/rust.

[MendyBerger]

Actually, I see the cpp generator doesn't seem to support async at all yet. I guess this would be a feature request for when it gets async support