wasi-http: Remove extra task when reading bodies (bytecodealliance#7475)

This commit is in the same spirit as bytecodealliance#7461 and is aimed at removing an
extra task when interoperating between WebAssembly and Hyper. The
intention here is to ensure that when data is ready immediately a
context switch between Tokio tasks isn't necessary. I'm still not sure
why this context switch would tank performance so much.

The goal of this commit is to remove the Tokio task associated with
reading bodies. Currently a task is spawned with two channels coming
out: one for frames and one for trailers. Instead the body is now read
directly when data frames are read and trailers are managed similarly.
The interactions here are somewhat nontrivial due to the management of
resources and how the body needs to jump from the body stream object to
the trailers object, but it's overall not the worst thing in the world.

Locally on Linux this takes a "hello world" component which starts out
by reading its HTTP body and then responds with a string from 6.7k rps
locally via `wasmtime serve` to 16.8k rps. This restores the performance
of wasi-http to be in-line with the historical SDK that Fermyon offered,
for example, that's based on a custom HTTP interface. Note that these
performance numbers use bytecodealliance#7461 as a baseline.

This commit has a downside, however, that if the incoming body is not
being subscribed to actively by the wasm then no activity will happen on
the host. This is a function of `poll` isn't being actively watched
unless `subscribe`'s result is being actively watched via a wasm call to
`wasi:io/poll`. Manual investigation of Hyper shows that this is
probably ok for now since bodies are primarily channel-based which seems
like the tasks doing the actual I/O are managed elsewhere. In that sense
this shouldn't cause any immediate impact, but Hyper's situation in the
future could change or my analysis could additionally be wrong. Given
the performance benefits here though I'd be tempted to push in favor of
this commit and handle "poll in the background" as necessary in the
future if needed.