Add Exhibitions identifier for linking purposes

[rcantin-w]

Currently the display object for an exhibition does not contain a simple way to identify them as Exhibitions, but we need that for the linkResolver on the FE to point to the correct URL.

We'd have to list all the different formats an Exhibition can be, but that's not very dynamic. We should add a field, a boolean would do, do determine whether or not this event should point to /exhibitions/uid instead of events/uid.

Definition of done

• Change is in prod
• New ticket is created for the FE to happen OR the FE work has happened (Amend link for Exhibitions in Events search wellcomecollection.org#12625)