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

`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 ` in the `cpp` generator with the same semantics as `c`/`rust`.