Allow CMake to take in a specific Python version

Issue:

If a users system has multiple python installations, when building the Python API, CMake will preference using the highest python version found in the system path.

This may be un-ideal if the user wants to preference a specific python version to build the API with e.g. ignore another installation. There currently isn't any interface or means to explicitly tell CMake to use a different Python version.

Fix:

Add the ability to pass a preferred Python version through a CMake config option. If passed, CMake will search for a python install that specifically matches the given version. Otherwise, default to the previous behaviour, taking the newest Python version it can find.