feat: add Wagi HTTP executor

[radu-matei]

ref #34

This draft PR is a prototype for running both Spin and Wagi HTTP executors.
It adapts the response composition logic from Wagi, and reuses the module instantiation from the underlying execution context.

A robust implementation would import the Wagi core as a crate and reuse the logic for headers, query parameters, and responses.

This should remain a draft until this implementation follows the Wagi spec closely.

WIth this change, a component of an HTTP application could define wagi as its executor:

name        = "spin-hello-world"
version     = "1.0.0"
description = "A simple application that returns hello and goodbye."
authors     = [ "Radu Matei <radu@fermyon.com>" ]
trigger     = "http"

[[component]]
    source = "target/wasm32-wasi/release/spinhelloworld.wasm"
    id     = "hello"
[component.trigger]
    route    = "/hello"
    executor = "spin"

[[component]]
    source = "wagi.wasm"
    id     = "wagi"
[component.trigger]
    route    = "/wagi"
    executor = "wagi"

Signed-off-by: Radu Matei radu.matei@fermyon.com

[radu-matei]

The one Wagi feature that will most likely not be part of this is a module defining its own routes using a _routes() function.

Because an executor doesn't have access to the underlying Wasm engine instantiation, there is no way for it to do that at startup and augment the route table (ref #19 ).

[radu-matei]

TODO: allow custom entrypoints in the component configuration.

[radu-matei]

Opened #69 to track this.