[iOS] Observe ReadingListModel changes for Reminder Notifications IPH

The TabBasedIPHBrowserAgent now implements the ReadingListModelObserver
interface to listen for relevant reading list changes.

Specifically, when a user manually adds an entry to the reading list,
the TabBasedIPHBrowserAgent will be able to (eventually) trigger a UI
command to display the Reminder Notifications Bubble IPH.

This CL extends the groundwork for the Reminder Notifications feature by
enabling the TabBasedIPHBrowserAgent to listen to the necessary reading
list events.

This addresses another part of the requirements for enabling the
Reminder Notifications feature on iOS.

Fixed: 390475544
Change-Id: I547082efbfafeba6702522664afe5012f499844b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6180083
Auto-Submit: Benjamin Williams <bwwilliams@google.com>
Commit-Queue: Benjamin Williams <bwwilliams@google.com>
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1408214}