cmake: add support for the python bindings

[lpapp-foundry]

What

As a host developer I would like to be able to build both the C++ and Python versions of MC in a fashion consistent with the core OpenAssetIO library so that I can vendor them within my own build system. Eg. Conan, etc... and be assured that the python and c++ builds are guaranteed to be in sync.

Why

Presently, if I need to vendor MC, I need two build phases and it is inconsistent with the core library.

ACs

• cmake optionally invokes traitgen for Python, outputting to the same dist tree as OpenAssetIO (don't use pip as we want to ensure the same traitgen is used as C++, whilst that isn't a managed dependency).
• The same traitgen is use as C++
• Documented differences with python -m pip install . in terms.
• OPENASSETIO_MEDIACREATION_PYTHON_SITEDIR is set to allow downstream usage to locate the build dir for python.

Original request

I would like to write a conan recipe for media creation, both for the C++ and Python parts.

Similarly, to OpenAssetIO, it would be good if the python interface was deployed "automatically".

In this case, this would mean:

1. Generating the python bindings.

  execute_process(COMMAND openassetio-traitgen ${PROJECT_BINARY_DIR}/traits.yml   
                -o ${PROJECT_BINARY_DIR}/python -g python
                COMMAND_ERROR_IS_FATAL ANY                                      
                COMMAND_ECHO STDERR)

or so.

2. cmake installs them into the usual sites-packages structure.

   OpenAssetIO has a OPENASSETIO_PYTHON_SITEDIR variable fro this purpose, so there could be a similar OPENASSETIO_MEDIACREATION_PYTHON_SITEDIR.

set(OPENASSETIO_MEDIACREATION_PYTHON_SITEDIR                                           
"lib/python${Python_VERSION_MAJOR}.${Python_VERSION_MINOR}/site-packages")  
if (WIN32) # Should not use 'bin' for Windows                                   
    set(OPENASSETIO_MEDIACREATION_PYTHON_SITEDIR "Lib/site-packages")           
endif()

install(                                                                        
    DIRECTORY package/openassetio_mediacreation                                        
    DESTINATION "${OPENASSETIO_MEDIACREATION_PYTHON_SITEDIR}"                   
    FILES_MATCHING PATTERN "*.py"                                               
) 

or something like that.

So that the conan recipe consumer does not need to do any extra work.

[foundrytom]

Reminder: when we do this, we need to cross-reference with how we do this in OpenAssetIO (where iirc pip just copes out of dist made by cmake as it always requires c++). I don't think that makes as much sense in this case where you can legitimately build python on its own. We don't want to involve cmake then.