Add custom vertex data support for third party plugins

[McManning]

Current API is specced as the following for Plugin:

    def add_custom_vertex_data_stream(self, id: str, size: int, callback):
        """
        Add a callback to be executed every time vertex data needs to be synced.
        The callback must return a pointer to the custom vertex data, aligned *to loops*
        for the given mesh.

        `def callback(mesh: bpy.types.Mesh) -> ctypes.void_p`

        Args:
            id (str):
            size (int):             Number of bytes in the data stream per loop index
            callback (callable):    Callable that returns a pointer to the data stream
        """
        pass

    def remove_custom_vertex_data_stream(self, id: str):
        """Remove a previously registered vertex data stream

        Args:
            id (str):
        """
        pass

Few thoughts:

• Currently requires loop alignment. Maybe it instead optionally can align to vertices so we don't need an extra loops[i].v mapping in C#?
• Need to better define how a third party plugin is fetching/generating that data. For Python-land it can be something like Numpy doing number crunching to spit out some values per vertex, but how does that look with other CDLLs?
• Will need to figure out some good method for stream reinterpretation on both the LibCoherence and Unity side of things. LibCoherence can at least do a memcmp for for each vertex's stride of bytes to determine if vertices need to be split or not. Unity - I guess can just reinterpret the byte stream however the 3rd party plugin wants?
• Still need specs on what this would look like for Unity. Are we letting people add data to the Mesh? Or use the data for something else (dump into a compute maybe...)? Weight paints would need to go into the Mesh directly.
• Space transforms? Is that just the responsibility of the 3rd party dev?