autosummary: event for dynamic context

As far as I can tell, there currently is no easy way to get additional dynamic context in autosummary templates, based on the element being processed.

If you require a specific example, I want to generate method and property autosummaries that also list all base classes where they are implemented.

For static data, there is autosummary_context, which is added to the context before the template is rendered. Just after that, I propose to emit a new event, autosummary-context. Callbacks to this event should be able to provide dynamic context based on the name, obj, parent, and static context being processed.

This can be achieved by a small addition along these lines after sphinx/ext/autosummary/generate.py:475:

         context: dict[str, Any] = {}
         if app:
             context.update(app.config.autosummary_context)
+            results = app.emit('autosummary-context', name, obj, parent, context)
+            for extra_context in results:
+                if extra_context:
+                    context.update(extra_context) 

         content = generate_autosummary_content(name, obj, parent, template, entry.template,
                                                imported_members, app, entry.recursive, context,
                                                modname, qualname)

It would also require the new event to be registered in the extension's setup() function, but no further changes.

The only alternative I can see is creating a custom extension based on autosummary.

Start at sphinx/ext/autosummary/generate.py around line 475 and inspect the extension's setup() function for event registration. Confirm how autosummary_context is prepared and how emitted callback results are handled. Done means the new autosummary-context event can provide dynamic context from name, obj, parent, and context for template rendering.