[iOS] Create the infrastructure to support provisional notifications.

This CL creates the infrastructure necessary to support provisional
notifications. Adds new methods and metrics to record provisional
notification events.

Bug: b/320721999
Change-Id: I04201d76e3b29d80b25a50734bd51f6a30553fd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5218272
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Marke Hallowell <wry@chromium.org>
Auto-Submit: Guillem Perez <guiperez@google.com>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1255404}