
This adds a callback to bNodeTreeType to check which socket types are valid for the tree type. Function has been implemented for the normal tree types, and can be implemented for custom node trees with python, by adding a `classmethod` to the tree. However, only builtin socket types are supported. This is relevant for T87049, but it also has the advantage that it is now clear which node trees support which sockets. Previously this was assumed to be known by all developers. Differential Revision: https://developer.blender.org/D10938