[Office] Implement sign-in

Add mojo methods to manage the sign-in process: `IsODFSMounted`
returns true if ODFS is mounted, which is linked to the user being
signed in. `SignInToOneDrive` is used to initiate the sign, and helps
distinguish via its callback a success (true), failure (false), or
still in progress.

Bug: b:251046528
Change-Id: Ie9872d550dbaf037aad90f320cc6dce1ec610e67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4033049
Reviewed-by: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1081337}