eclass-overlay: Add federated-service user and group

Set up user and group for running the Federated Computation service in
minijail.

BUG=chromium:1020051
TEST=profiles/base/accounts/display-accts.py

Change-Id: If4d103e678fa8058f925dc092870518115326a48
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2355371
Tested-by: Xinglong Luan <alanlxl@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>