[iOS] Trigger identity confirmation snackbar from Authentication Flow

The identity confirmation snackbar should be shown after a successful
account switch.

Before this CL, the snackbar was triggered in the signin completion
block in the account menu coordinator, which resulted in the snackbar
trigger to be gone if there was a profile switch.

This CL adds a new PostSigninAction dedicated for showing this
snackbar, and moves the logic from the account menu to the
authentication flow to guarantee it is handled in case there was a
profile switch.

Fixed: 399857552
Change-Id: If468f21e8d5018bd1a1ef7ea293c3ab3100b659e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6366756
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Jood Hajeer <jood@google.com>
Cr-Commit-Position: refs/heads/main@{#1434812}