The main challenge is to avoid dangling pointers. Currently, the lifetime of socket
declarations is somewhat unbounded (at least we didn't restrict it explicitly yet).
Therefore, storing non-owning pointers in it tricky. For ID pointers one could
potentially use the foreach-id iterator to update pointers in declarations as well,
but that's a bit out of scope and might not be the right solution anyway, since it's
not obvious that all node declarations are reachable from IDs stored in `bmain`.
The solution now is to use a callback that retrieves the right ID pointer when it
is used. The important thing is that the callback does not capture any potentially
dangling pointer either.
Pull Request: https://projects.blender.org/blender/blender/pulls/107179
f6ec11741c