kiosk: Add IWA Kiosk user type and profile load

This CL introduces a separate user type in UserManager for IWA Kiosk.
It also implements a LoadProfile step during kiosk session launch.
Design doc: go/cros-kiosk-iwa-dd

Bug: 361019018
Change-Id: I63a7add53f3b91f318a2607e603a24016e6a03a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5890817
Reviewed-by: Igor <igorcov@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sergii Bykov <sbykov@google.com>
Cr-Commit-Position: refs/heads/main@{#1364120}