Use unasync approach

[karpetrosyan]

We have over 800 lines of code in our Client class.
We also have an AsyncClient, which is the same Client but with async/await syntax, and we should literally write the same code twice.

We can consider including the same unasync.py file from httpcore](https://github.com/encode/httpcore/blob/master/unasync.py) in our CI to ensure that Client and AsyncClient are always in sync.

It's also more robust and secure because issues like #3023 and #3042 are eliminated.

[rafalkrupinski]

Would it be an option to implement the Client around generators, which works with both sync and async, and then make Client and AsyncClient facades?

[karpetrosyan]

Sorry for delayed answer, I think unasync is much easier and more familiar to the maintainers. Also, i don't know if the generators would work for the tests.

[rafalkrupinski]

I was just hinting to a potential solution, it's really a matter of preference, generated code vs architecture. Httpx is already using generators for auth flows.