wasm: add wasm32-wasip2 networking support (#7933)

Motivation

This adds networking support for the `wasm32-wasip2` target platform, which
includes more extensive support for sockets than `wasm32-wasip1`.

Solution

The bulk of the changes are in https://github.com/tokio-rs/mio/pull/1931.  This
patch mainly tweaks a few `cfg` directives to indicate `wasm32-wasip2`'s
additional capabilities.

Note that this is a draft PR until until
https://github.com/tokio-rs/mio/pull/1931 and
https://github.com/rust-lang/socket2/pull/639 have been include in stable
releases of their respective projects.

Also note that I've added a `wasm32-wasip2` target to CI and triaged each test
which was previously disabled for WASI into one of three categories:

- Disabled on both WASIp1 and p2 due to not-yet-supported features such as multithreading
- Disabled on p1 but enabled on p2
- Disabled on p1 and _temporarily_ disabled on p2 due to `wasi-libc` bugfixes which have been merged but not yet included in a Rust release.  I'll open an issue to re-enable them when the fixes land in Rust.

Future Work

In the future, we could consider adding support for `tokio::net::lookup_host`.
WASIp2 natively supports asynchronous DNS lookups and is single threaded,
whereas Tokio currently assumes DNS lookups are blocking and require
multithreading to emulate async lookups.  A WASIp2-specific implementation could
do the lookup directly without multithreading.

WASIp2 also supports single-threaded, asynchronous file I/O, timers, etc.  We
could either support those directly or wait for WASIp3's multithreading support,
in which case most of `tokio::fs` (as well as `tokio::net::lookup_host`, etc.)
_should_ work unchanged via `wasi-libc` and worker threads.

Currently, building for WASIp2 requires RUSTFLAGS="--cfg tokio_unstable"`.  Once
we have a solid maintenance plan, we can remove that requirement.

.github/workflows/ci.yml

@@ -1117,6 +1117,32 @@ jobs:
           CARGO_TARGET_WASM32_WASIP1_THREADS_RUNNER: "wasmtime run -W bulk-memory=y -W threads=y -W shared-memory=y -S threads=y --"
           RUSTFLAGS: --cfg tokio_unstable -Dwarnings -C target-feature=+atomics,+bulk-memory -C link-args=--max-memory=67108864
 
+  wasm32-wasip2:
+    name: test tokio for wasm32-wasip2
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: ${{ env.rust_stable }}
+          targets: wasm32-wasip2
+
+      - name: Install cargo-nextest, wasmtime
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest,wasmtime-cli
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: test tokio --target wasm32-wasip2
+        run: cargo nextest run --target wasm32-wasip2 --features net,macros,rt,io-util
+        working-directory: tokio
+        env:
+          RUSTFLAGS: --cfg tokio_unstable
+          CARGO_TARGET_WASM32_WASIP2_RUNNER: wasmtime run -Sinherit-network
+
   check-external-types:
     name: check-external-types (${{ matrix.os }})
     needs: basics