Add update tags directly to bNodeTreeInterface to enable tagging by API.

Calling an API function after the node panels patch does not internally
tag the node tree with `NTREE_CHANGED_INTERFACE` any more, because the
node tree is not directly accessible from `bNodeTreeInterface`. Before
node panels the API functions for interfaces could tag the tree directly
for later update consideration, which now requires explicit tagging
calls.

The fix is to add a flag and mutex directly to `bNodeTreeInterface`, so
API methods can tag after updates. This mostly copies runtime data
concepts from `bNodeTree`. The `ensure_interface_cache` method is
equivalent to `ensure_topology_cache` and should be called before
accessing `interface_inputs` and similar cache data.

@@ -1109,2 +1131,4 @@
     parent->insert_item(new_socket->item, position);
   }
+
+  tag_items_changed();

@@ -1270,27 +1312,70 @@ void bNodeTreeInterface::foreach_id(LibraryForeachIDData *cb)
   item_types::item_foreach_id(cb, root_panel.item);
 }
 

@@ -396,0 +414,4 @@
+   */
+  void tag_items_changed();
+
+ protected:

@@ -27,0 +27,4 @@
+  /**
+   * Keeps track of what changed in the node tree until the next update.
+   * Should not be changed directly, instead use the functions in `BKE_node_tree_update.h`.
+   * #eNodeTreeChangedFlag.

@@ -27,0 +44,4 @@
+  Vector<bNodeTreeInterfaceSocket *> outputs_;
+
+ public:
+  blender::Span<bNodeTreeInterfaceItem *> items() const

@@ -1049,1 +1066,4 @@
   item_types::item_read_data(reader, this->root_panel.item);
+
+  this->runtime = MEM_new<blender::bke::bNodeTreeInterfaceRuntime>(__func__);
+  tag_missing_runtime_data();