Add async call stacks to the public API, use it in wasi:http (bytecodealliance#13510)

* Enable reflection on component async call stacks

This commit adds a few new APIs to the surface area of the `wasmtime`
crate as well as reorganizes some internals. Namely:

* `StoreContextMut::async_call_stack` - yields an `Iterator` of the call
  stack of async tasks in the component model to understand the current
  call graph.

* `{Typed,}Func::{start,finish}_call_concurrent` - these new APIs allow
  splitting apart the previous `call_concurrent` function into discrete
  steps to learn about the task being created, notably the
  `GuestTaskId`. The previous `call_concurrent` is reimplemented in
  terms of these functions.

* Methods requiring the `component-model-async` Cargo feature are now
  under the `concurrent` module in `wasmtime` to cut down on `#[cfg]`
  required.

This is all intended to address the concerns of WebAssembly/WASI#918,
WebAssembly/WASI#919, and WebAssembly/WASI#920. This isn't plumbed into
wasi-http yet, but that'll come as a

* Thread request IDs through `wasi:http` handling

This commit uses the previous commit to connect a generic, host-defined,
request ID and provide a strong connection to the `GuestTaskId` that's
being used to serve that request. This can be used by `wasi:http`
handlers to provide a strong correlation between outgoing requests, for
example, and incoming requests.

* Update expansion tests

* Update crates/wasmtime/src/runtime/component/concurrent/func.rs

Co-authored-by: Joel Dice <joel.dice@akamai.com>

* Fix doc links

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Co-authored-by: Joel Dice <joel.dice@akamai.com>

Author: alexcrichton