HTTP middleware using WASIp3

[itowlson]

Fixes #3291.

This currently includes #3383; once that lands I will rebase. For now just ignore the wit directory. Done!

Example usage: https://github.com/itowlson/spin-middleware-terrifying-nonsense

Setting it up in spin.toml:

[[trigger.http]]
route = "/..."
component = "middleware-terrifying-nonsense"
components.middleware = ["middlybiddly", "middlybiddly2"]
executor = { type = "wasip3-unstable" }

Implementing a middleware component in Rust (today: we'd presumably provide a wrapper for the onbound handle import):

#[spin_sdk::http_wasip3::http_service]
async fn handle(mut request: Request) -> impl IntoResponse {
    munge(&mut request);
    wasi::http0_3_0_rc_2026_01_06::handler::handle(request.into_request().unwrap()).await
    // could process the response here if needed
}

Needs lots of renaming, tests, etc. but anyway getting it into the system. (and yes, yes, I'll squash the commits)