HySDK - lightweight client library for easier proxy tool integration

[tobyxdd]

Many third-party proxy tools that support Hysteria right now either copy and modify code from this repo or write it themselves from scratch, leading to inconsistent quality and behavioral disparities across implementations. This will also hinder new feature rollouts & protocol upgrades.

One of the root causes is that Hysteria's current code cannot be easily invoked as a library. The core is coupled with other stuff, and there is no documentation. We should clean up the code and wrap the client core into an easy-to-use, well-documented library.