feat: aiohttp request body

[revimi]

Partially fixes #220
In case if the request body has been read at least once, we have it in the request._read_bytes attribute. It allows us simply access it from non-async code.

[untitaker]

@revimi could you add the following:

• Some test in tests/integrations/aiohttp
• Some sort of placeholder value for body in case we could not read it, such that a user can distinguish between "there is no body" and "body was not read". I believe this is only a check of request.body_exists

[revimi]

@untitaker request.body_exists returns True even if it was GET request without the body. I used request.can_read_body. It returns True if the request has some data in the body but we haven't tried to read it.

[revimi]

I'll add tests a bit later