Allow users to define required version in cmake

[rolflussi]

Proposed change

Would it be possible to provide a cnats-config-version.cmake file to allow any cmake based project to define the required version of this library within the find_package command?

Use case

Any cmake project can define the required version in its CMakeLists.txt file like this:

find_package(cnats 3.9 REQUIRED)

Contribution

I already have a first implementation on a fork, so I would be happy to open a pull request if you are interested.

[levb]

@rolflussi please feel free to open a PR on this! Will be much appreciated!