New Stream API Support

[yhirose]

This PR is trying to implement #2269. It requires C++20 compiler or later which supports C++ coroutines.

[yhirose]

@ngxson the only thing of this new stream API is that it requires C++20 which supports C++ coroutines.

Here is README.

[yhirose]

@ngxson I discarded my first implementation for C++20, then implemented the stream API which works with C++11 according to your request at #2269.

[ngxson]

Thanks, indeed C++11 or 17 is better, but not necessarily as we already moved cpp-httplib to a dedicated unit in llama.cpp. We have some other vendors compiled with C++17 too, while the main project still uses 11, so it's probably fine even if internally you want to use C++20 features

[ngxson]

Just read through the documentation, this is more than enough for me. Thanks again 😄