Currently node group assets are supported, but using them by dragging from the asset browser is cumbersome. This patch adds all node group assets from user asset libraries and the current file libraries to the add node search menu and the link drag search menu. Node groups added through the search will have their "options" hidden, meaning the data-block selector is displayed. This helps keep the UI clean, and the selector shouldn't be necessary anyway. To make that possible, metadata like the node tree type and its inputs and outputs has to be saved in the file. This requires re-saving the files that contain the assets with the patch applied. The node add search operator is moved from Python to C++ to ease development and allow more flexibility. It supports a tooltip that gives the description of assets. Currently the node groups are added with the asset system's existing "Append & Reuse" behavior. It's likely that linking should be possible in the future too, but for now the idea is to use the more foolproof option that doesn't create dependencies between files. Because loading assets can potentially take a long time, the search menu refreshes its items as new assets are loaded. However, changing the search field is necessary to see the update. Differential Revision: https://developer.blender.org/D15568