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This adds a DBus service provider to enable ChromeOS daemons to call
into Ash to start the in-session user authentication dialog. The dialog
will be implemented in Ash.

Currently the only expected client is u2fd, which needs this UI flow
in WebAuthn.

TEST=Used u2fd to call UserAuthenticationServiceProvider and verified
     it received the call by logging.

Cq-Depend: chromium:2291256
Bug: b:144861739
Change-Id: I47c6b88f2eb4484cef197a009e6ad5bbc4b3e5c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2291258Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789336}