ASGI support

### Is your feature request related to a problem?

As far as I know most async Python web frameworks nowadays use ASGI with servers like Uvicorn. However, as far as I can tell aiohttp server does not support ASGI.

### Describe the solution you'd like

It would be nice if aiohttp could support ASGI, so that it can benefit from features provided by ASGI web servers (see for example https://github.com/aio-libs/aiohttp/issues/5999#issuecomment-930626734).

There may also be some performance benefits to ASGI over WSGI, but I don't know enough about the internals of how aiohttp interfaces with WSGI to make specific claims about this.

### Describe alternatives you've considered

Aiohttp isn't fundamentally broken without this feature so doing nothing is always an alternative.

### Related component

Server

### Additional context

There was pervious discussion of this in 2018 (https://github.com/aio-libs/aiohttp/issues/2902). The final decision there seemed to be to wait and see. Since then ASGI has become much more stable and much more widely used, so I think it is time to revisit this decision.

### Code of Conduct

- [X] I agree to follow the aio-libs Code of Conduct