Generalize related entitites in more-info dialog like the Group members

[Alsatian67]

Describe the enhancement

When using the Group integration https://www.home-assistant.io/integrations/group/ ,

The entity information dialog displays a nice list of member widgets :
https://www.home-assistant.io/blog/2025/08/06/release-20258/#control-individual-members-of-a-group

It would be nice to generalize this group feature to each entity type, allowing to mix entity and subentity domains.

Use cases

I have a cover (main entity), related with a lock entity (locks the cover) and a switch entity (enable/disable auto-movement depending on the sun position).

If I could declare the lock and the switch as "member" or "related" to the cover
(see Group integration special _attr_extra_state_attributes["entity_id"] implementation),

it would be very easy to build a configuration dialog for this cover, where I can access the config entities of the cover, within the entity information dialog. No need to create a device for each cover or custom cards.

Visual examples

See 08/2025 release announcement for the group integration members :
https://www.home-assistant.io/blog/2025/08/06/release-20258/#control-individual-members-of-a-group

Anything else?

more-info "member entities" (or "related entities") should be selectable from the UI.

In case of Group integration groups (or other integration providing entity_id as extra attribute):
This UI selection of related entities can be pre-filled at the creation of the entity, and editable.
Then they will be removable (Sometimes the Group user don't want to see the members in the more-info dialog).