Support for Async Generators

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

Currently encountering an asynchronous generator ends the stack. That is because they are [not introspectable](https://github.com/python/cpython/issues/76991).

### Describe the solution you'd like

I would like the ability to view a full stack containing Async Generators.

### Describe alternatives you've considered

I made [a proof of concept](https://github.com/catwell/aio-mischief) to show it can be done with ctypes. Not sure you would consider this in aiomonitor proper.

### Additional context

I had that need while debugging code using [httpx](https://www.python-httpx.org/) which is basically a big stack of Async Generators.

### Code of Conduct

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