Forced notification when a node is created or updated via API (not via UI forms)

Problem/Motivation

I'm working on a site where a specific node type can be posted to groups. These nodes start life unpublished, with a date range field defining the window of time they should be published. I need to automatically notify all group members when one of these nodes is published.

#3076248: Notification toggle on save and edit gives us the possibility to conditionally send group notifications when editing a node.

However, because it all works at the Form API level, if you have something programmatically updating (and in this case, publishing) nodes, there's no way to twiddle those checkboxes and generate the emails.

Proposed resolution

Not entirely sure what's best for this. If there was some code at the hook_ENTITY_TYPE_update() level that looked for the presence of a magic property, I could force this to work with something like:

        // Array of Group entity IDs to notify with this node update:
        $node->group_notify[] = $group->id();
        $node->setNewRevision(TRUE);
        $node->setPublished();
        $node->save(); 

Or maybe there should be both an array if you want specific Group IDs (above) or a bool if you want everything:

        $node->group_notify_all = TRUE;

Remaining tasks

1. Decide on how this should work.
2. Implement.
3. Testing.
4. Automated tests?
5. Reviews / refinements.
6. Commit.

User interface changes

None. This would be entirely at the Entity API level.

API changes

TBD.

Data model changes

None.