Improve handling of runtime defined python RNA properties. Mainly:
* Add `get_transform` and `set_transform` new callbacks.
These allow to edit the value, while still using the default
(IDProperty-based) storage system.
* Read-only properties should now be defined using a new `options` flag,
`READ_ONLY`.
* `get`/`set` should only be used when storing data outside of the
default system now.
* Having a `get` without a `set` defined forces property to be
read-only (same behavior as before).
* Having a `set` without a `get` is now an error.
* Just like with existing `get/set` callbacks, `get_/set_transform`
callbacks must always generate values matching the constraints defined
by their `bpy.props` property definition (same type, within required
range, same dimensions/sizes for the `Vector` properties, etc.).
* To simplify handling of non-statically sized strings, the relevant
RNA API has been modified, to use `std::string` instead of
(allocated) char arrays.
Relevant unittests and benchmarking have been added or updated as part
of this project.
Note: From initial benchmarking, 'transform' versions of get/set are
several times faster than 'real' get/set.
Implements #141042.
Pull Request: https://projects.blender.org/blender/blender/pulls/141303
