Switch MC to using `openassetio-traitgen`

[foundrytom]

What

Once openassetio-traitgen is available in PyPI, convert the traits in the repo to .yml definitions, and build with openassetio-traitgen as part of setup.py.

Why

Manual authoring of Trait/Specification code is deprecated as it is prohibitively slow.

Notes

Requires https://github.com/OpenAssetIO/OpenAssetIO-TraitGen/issues/2

We currently have the strategy that we "trust" traitgen code through its own testing, so there isn't need to write extensive tests for autogenerated classes.

It's to be expected that the resulting Python namespace will change.

Be good to hard code the yaml hints to the JSON schema in github for traitgen.

• Timeline, Track and Clip traits -> a timeline namespace
• LoatableContent -> a content namespace(?)

ACs

• Uses PyPI OpenAssetIO-TraitGen
• python -m pip install . installs an importable openassetio_mediacreation module in Python 3.7+
• Test coverage of the resulting module similar to OpenAssetIO test_imports.
• MC traits/specifications defined in yaml.
• No yaml-generated python code in the repo.
• User and developer documentation etc. updated
• TraitGen pinned to minor semver (ie, pick up patches).
• The source YAML file is included in the installed module at the root.