ci(wasip2): document and test wasm32-wasip2 support (Postgres + MySQL)

[ricochet]

SQLx already runs on wasm32-wasip2 with no library changes: the standard runtime-tokio feature works there because Tokio's net driver builds on WASI via mio. Pooling, LISTEN/NOTIFY, timeouts, and rustls TLS all work. Building for the target requires --cfg tokio_unstable and a current-thread runtime (WASI has no threads in wasip2 target).

This exercises that support:

• README: add a WebAssembly (wasm32-wasip2) section covering the runtime-tokio setup, the two target requirements, what works, and the remaining limitations (hostname lookup needs --allow-ip-name-lookup, native-tls is unavailable, MySQL caching_sha2 over plaintext needs
  mysql-rsa).
• examples/wasip2/{postgres,mysql}: runnable pool examples that do a real round-trip — create a table, insert rows in a transaction with bind parameters, read them back ranked, and aggregate. A shared .cargo/config.toml sets --cfg tokio_unstable so a plain cargo build --target wasm32-wasip2 works.
• CI: a wasip2 job builds both examples and runs them under Wasmtime against Postgres and MySQL service containers, asserting the query output to catch wasm-specific decode regressions.

[ricochet]

CI on main is failing. Created a PR to fix: #4311

[abonander]

As I mentioned in #4058 (comment) I don't really see the point in maintaining support for an unstable or tier 2 compilation target. I feel this even more strongly now, because there looks to be significant changes coming to I/O (and especially async I/O) in wasip3: https://wasi.dev/releases/wasi-p3#what-changed-in-each-interface

This seems likely to be a moving target for a while yet, and I just don't have the bandwidth to spend time on issues related to this.

[ricochet]

Understood on maintainer resources and time. If there is ever renewed interest, I would gladly jump in as a codeowner for wasi support. wasip3 itself is stable and launched (wasip2 is also stable). I've also already tested sqlx with wasip3 and it does not require any changes to work.

This is the end-user-friendly version of the WASI 0.3 release notes that I wrote.