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)

[itowlson]

Updated to rework OCI precomposition. The way I did it before resulted in two code paths which were essentially the same but couldn't be exactly the same. Now there is only one code path and it is exactly the same as itself.

I am getting increasingly fearful that I am getting lost in a labyrinth of my own changes and although it all seems to work for me I would really value somebody else looking at it and kicking the tyres because what if I am not testing it right. I was sure that this last rework had broken something, and yet seemingly not, which makes me worry whether I am testing what I think I'm testing.