Versioning generation behaviour.

[elliotcmorris]

Add functionality to generate versioned traits in trait-gen.

All of the trait-view versions for a particular trait should be generated into the same file.

Json schema will need to be updated.

Acceptance Criteria

Follow the structure outlined in OpenAssetIO/OpenAssetIO-MediaCreation#90

Apart from these changes:

The unversioned default trait should be an identical copy of the v1 trait.
The unversioned default trait should have a deprecation warning. In C++ this is via the [[deprecated]] attribute on the class. In python it's on the constructor, and use the pythons warnings module.