Convert library to header only (qsbr & art)

[thompsonbry]

With the refactor to make the Key a template argument, we should not go to a header only library. Refactor the CMake and CI as required to achieve this outcome. Provide guidance if required for customers about how to consume the resulting library.

[laurynas-biveinis]

QSBR should not be affected by this and remain a compiled library. Likely this means that the interface between QSBR and the rest needs to be formalized.