[iOS] Switch ContextualPanel to use pointers not values for subclassing

Before, when the individual model classes returned subclasses of
ContextualPanelItemConfiguration, any additional data was lost. Now,
they return pointers to item configurations, so they can return pointers
to configuration subclasses. These pointers will be provided to the
individual UI classes later on.

Bug: 316369990
Change-Id: I0a39f1b4849aeb962d7d627fddd68d2aef76fc40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5400743


Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/main@{#1282774}