[iOS] Add a method to record carousel scrolls within a cell in the feed

This CL adds a method to record scrolls on in-cell carousels in
the feed on UMA. It is used by homestack and will be especially
useful once homestack is fully rolled out.

Works in conjunction with cl/681624191 and crrev.com/i/7724315.

Bug: 358163731
Change-Id: Icf13a396ef21990701792d766fd36005cfbd14f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5905539
Commit-Queue: Ginny Huang <ginnyhuang@chromium.org>
Auto-Submit: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1363691}