Enable async support for Flask

[abdnh]

This enables async request handlers for Flask. Useful for the Svelte ports of the editor and reviewer (#4029, #4289). I already use this in my PR, but I thought it'd be nice to extract it into its own PR to share.

To give a usage example from the reviewer port, the PlayAVTags RPC can be implemented like this:

async def play_avtags():
    from anki.frontend_pb2 import PlayAVTagsRequest
    from anki.template import av_tags_to_native
    from aqt import gui_hooks, mw
    from aqt.sound import av_player

    def on_av_player_did_end_playing(*args: Any, **kwargs: Any) -> None:
        def task() -> None:
            if av_player.queue_is_empty():
                request_handler.set_result(None)
                gui_hooks.av_player_did_end_playing.remove(on_av_player_did_end_playing)

        # Allow time for audio queue to update
        mw.taskman.run_on_main(lambda: mw.progress.single_shot(100, task))

    def callback(request_handler: AsyncRequestHandler) -> None:
        gui_hooks.av_player_did_end_playing.append(on_av_player_did_end_playing)

    request_handler: AsyncRequestHandler[None] = AsyncRequestHandler(callback)
    req = PlayAVTagsRequest.FromString(request.data)
    av_player.play_tags(av_tags_to_native(req.tags))
    await request_handler.run()

    return b""

The frontend can then await the request in case "Wait for audio" is enabled.