Part 2/3 of #109135, #110272
Defines the RNA API for the new node tree interfaces.
The bulk of the RNA definition lives in `rna_node_tree_interface.cc`. The legacy socket interfaces remain in place and will be removed later.
Since the socket items share the `bNodeSocketValueXXX` structs with the `bNodeSocket` types they also use the same RNA. The `rna_def_node_socket_interface_subtypes` function is exposed so that when defining the interface RNA it can use the same code as the regular socket RNA.
Pull Request: https://projects.blender.org/blender/blender/pulls/110952
ea3d85b9d7