Attach JS and CSS library

That's an interesting use case; however, it falls outside the scope of this module. If you wanted to implement with custom code, you should be able to accomplish that using hook_form_alter() and hook_preprocess_block().

It's so simple, if I create a patch, could you add it to the module? I think that's the case. Avoid using too much hook.

@lazzyvn You're welcome to post a patch; however, as I mentioned, this falls outside the scope of this module, so it won't be committed.

In this case I think I will create a new module with some functions that the block layout builder needs: add attribute, class , attach library, ...