Part 1/3 of #109135, #110272
Adds a new DNA structure for defining node group interfaces without
using `bNodeSocket` and with additional node UI item types.
Node group interfaces are organized as a hierarchy of "items", which
can be sockets or panels. Panels can contain both sockets and other
panels (although nested panels beyond the root panel may be disabled to
avoid complexity on the user level).
Sockets can be added to the interface in any order, not just the
conventional outputs..inputs order. Sockets can be marked as both input
and output, generating 2 sockets on node instances.
The C++ API in the DNA struct allows manipulating the interface
declaration by adding and removing items, moving them inside the
interface or into a different panel.
Pull Request: https://projects.blender.org/blender/blender/pulls/110885
