wasi-http: Manage outgoing connections with a callback on WasiHttpCtx

[elliottt]

Currently all outgoing connections assume http 1, and a specific tls library. We need to refactor this so that the logic for constructing the hyper::SendRequest<_> and spawning the connection handling task are done by a callback defined on the WasiHttpCtx instead.

[dicej]

I'm going to work on this today and try to get it backported to Wasmtime 14 before it's released.