Fix bug in coroutine scheduling. Add ByteStream implementation and simplify...

Fix bug in coroutine scheduling. Add ByteStream implementation and simplify Python symbol visitor. (#1633)

* Fix bug in coroutine scheduling. Add ByteStream implementation and simplify Python symbol visitor.

* Bug in coroutine scheduling:

In the previous iteration, the runtime crate aws-smithy-http-server-python
was exposing the python application implementation as a struct, which
was wrapped by the codegenerated App to allow to dynamically building
the router.

This caused scheduling of coroutines (handlers prefixed with async def)
to block becuse we were passing the Python eventloop of the parent
process that was stored pre-fork().

This commit changes the runtime PyApp to become a trait, allowing us to
dynamically build the router post-fork() and with the right event loop.

This change also allowed us to remove a bunch of unnecessary Arc(s).

* Add ByteStream implementation

Implementation of a ByteStream type for Python that can be roughly used like this:

let b = await ByteStream.from_path("file.txt")
async for chunk in b:
    print(chunk)

Implement futures_core::stream::Stream for Python ByteStream wrapper.

The BytesStream implementation in python can now use a sync Mutex from
parking_lot because we are now using pyo3_asyncio::tokio::local_future_into_py()
to read a chunk, which supports !Send futures.

This allows to simply forward the implementation of Stream (poll_next()
and size_hint()) directly to our inner SDK ByteStream.

* Simplify Python symbol visitor

Inherit from Symbol visitor and override just what is needed to swap Python complex
types.

Signed-off-by: Bigo <1781140+crisidev@users.noreply.github.com>