CMake support

[ANaumann85]

Adds

• the folder cmake to the root. This folder contains two configuration files
  • UmbridgeConfig.cmake allows to use the repository directly
  • UmbridgeConfig.cmake.in will be installed when using cmake (and the corresponding CMakeLists.txt)

This way, cmake users can install the library using

cmake -S ./cmake -B <buildFolder>
cmake --install <buildFolder> --prefix <destFolder>

As this lib provides json.hpp and httplib.h, I suggest to reorganize the code a little bit to

cpp/include/umbridge/[umbridge | json | httplib].h
cpp/README.md

such the client would include umbridge/umbridge.h. This way you can ship your own json and httplib without getting conflicts with system provided json.h and httplib.h

What do you think?