Add account capability for cros genai features

This is used to limit minor users from using some GenAI features on
ChromeOS.

Server side CL: http://cl/718380238

Bug: b:379786952
Test: CQ
Change-Id: I81cb47f7bf012ed529236f977b0907f844fece7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6248617
Reviewed-by: Yi Chou <yich@google.com>
Commit-Queue: Howard Yang <hcyang@google.com>
Reviewed-by: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1419007}