Add hook_custom_elements_entity_prepare_build

Per #3440446-3: Add prepareBuild step similar to entityprepareView → :

related, it would be nice to add some hooks like hook_custom_elements_entity_build and hook_custom_elements_entity_prepare_build - but that seems like a dedicated issue we should probably better add as follow-up, so we can focus on making good hooks there.

So, adding.

I think hook_custom_elements_entity_build ~= the existing 'custom_element_entity' alter hook.

I don't have an opinion on hook_custom_elements_entity_prepare_build ; just adding this issue for later thinking about.