Revive the client-sdk-cpp repo and make it compilable and runnable

[xianshijing-lk]

The PR does:

• Fixed the CMakelist.txt to make it compilable
• Fixed README.md
• Fixed the example
• Added build.sh to make it easier to clean | build

[ladvoc]

@xianshijing-lk, it looks like the CI workflow doesn't work anymore—should we address that here or in a separate PR? Also, I see this repo builds the Rust SDK from source rather than pulling in a prebuilt dylib. Do you have any thoughts on which you prefer?

[xianshijing-lk]

@xianshijing-lk, it looks like the CI workflow doesn't work anymore—should we address that here or in a separate PR? Also, I see this repo builds the Rust SDK from source rather than pulling in a prebuilt dylib. Do you have any thoughts on which you prefer?

I added the CI since I think that should be easily done, but the Linux bot is more complicated than I thought. And it looks like the last patch gets the bot running.

The current solution builds a static liblivekit-ffi.a and can be linked to any executable target. C++ Is a build from source world, lots of project might require building the code from source unless the code is too large or complex.

But I do think we probably want to use a pre-built liblivekit-ffi.a here, as it simplies the problems of building Rust and WebRTC

[ladvoc]

LGTM ✅