feat(p3): add client interface

[rvolosatovs]

Introduce the client interface designed for wasip3, which is at feature-parity with what's currently available in main for wasip2 with a few additions

bytecodealliance/wasmtime#12174 contains mostly-complete host implementation of the interface in the proposed shape with a passing test (adapted directly from existing wasip2 test). The interface in the PR is an in-progress superset of the interface proposed in this PR.

This is by no means complete and is intended to just be the first step with more PRs to follow as I validate the design by continuing the host implementation in Wasmtime as well as a Python guest implementation utilizing these (have not started on Python work yet)

refs WebAssembly/wasi-sockets#100
refs WebAssembly/wasi-sockets#104

[badeend]

A recap of what Roman & I have discussed out-of-band:

• The 0.2 wasi-tls interface uses wasi-io's error type to communicate failure. The proposed interface in this PR doesn't have any error handling yet. We could add a separate types interface in wasi-tls with e.g. a verbatim copy of the old wasi-io error type.
• Compared to the 0.2 interface, Roman's proposal turns certificate verification from a host concern into a guest concern. I can see that as a valuable advanced option for the future, but I don't think that should be the default.
• The proposed interface also adds alpn & cipher-suite functions. For the sake of true feature parity with 0.2, I'd prefer to postpone these to a follow-up PR because there will be some implementation challenges.
• Regarding the placement of the streams; the proposed interface is strictly speaking the most correct thing to do from a type state POV. But it also results in some gnarly function signatures. In the sockets interfaces we had the same problem and eventually went with removing the I/O streams from the connect method and instead added separate send & receive methods. Maybe we can do something similar in wasi-tls.

All of this combined then results in an interface closer to:

interface types {
    resource error {
        to-debug-string: func() -> string;
    }
}

interface client {
    resource connector {
        constructor();

        /// Set up the encryption stream transform.
        /// This takes an unprotected `cleartext` application data stream and
        /// returns an encrypted data stream, ready to be sent out over the network.
        /// Closing the `cleartext` stream will cause a `close_notify` packet to be emitted on the returned output stream.
        send: func(cleartext: stream<u8>) -> tuple<stream<u8>, future<result<_, error>>>;

        /// Set up the decryption stream transform.
        /// This takes an encrypted data stream, as received via e.g. the network,
        /// and returns a decrypted application data stream.
        receive: func(ciphertext: stream<u8>) -> tuple<stream<u8>, future<result<_, error>>>;

        /// Perform the handshake.
        /// The `send` & `receive` streams must be set up before calling this method.
        connect: async func(this: connector, server-name: string) -> result<_, error>;
    }
}

The connect method here doesn't return anything yet. The idea is that in a later version, it will return a connection resource through which the guest can read back the negotiated TLS version, ALPN id, cipher suite, server certificate, etc.

[rvolosatovs]

@badeend I've applied your suggestion and did a little fixup in 52e5f81

The interface is implemented in bytecodealliance/wasmtime#12174

[badeend]

Awesome! Thanks.