Add wasip2 support

[tomasol]

This change enables calling unary RPCs and response streaming from binaries built for the wasm32-wasip2 target.

[devashishdxt]

Thanks for working on this. I'm not sure if this crate is the right place to add support for this. Anyway, can you please add some tests?

[tomasol]

I wasn’t sure either, but started exploring it and ended up completing it. No pressure, totally understand if it’s not the right fit.I'll add tests running on wasmtime.

[tomasol]

The tests fit nicely, essentially I just replaced an annotation. The whole change that enables wasip2 is in call and it uses most of other modules like response_body, body_stream etc. Let me know if you have any concerns or questions!

[devashishdxt]

@tomasol Can a standard gRPC client run directly in a WASI environment?

I'm a little confused about why gRPC-Web would be necessary. I was under the impression that gRPC-Web is a workaround specifically for web browsers. Does WASI's networking model have similar constraints that prevent a standard gRPC implementation from working?

[tomasol]

gRPC proper isn't currently supported in WASI, see this issue for context. While it's technically possible to send gRPC messages over HTTP/1, that requires non-standard server configs.

Even once full support lands in WASI runtimes, a gRPC-Web library could still be useful. For example, a WASI app might want to target gRPC-Web to integrate with existing infrastructure (e.g., sharing a port already exposed for gRPC-Web traffic).