Add device identification data for attestation-based registration

Add a new message and a field in the certificate-based registration
data that allows us to provide device identification during
registration.

Bug: b:153503823
Test: None
Change-Id: If6246345a52f79114622fb8d6c3cfc35fe81f635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141216Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Auto-Submit: Yves Arrouye <drcrash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758236}

components/policy/proto/device_management_backend.proto

@@ -185,7 +185,16 @@ message DeviceRegisterRequest {
   // domain from token.
   optional string expected_enrollment_domain = 17;
 
-  // Next id: 18.
+  // Identification of the device that is not already available.
+  optional DeviceRegisterIdentification device_register_identification = 18;
+
+  // Next id: 19.
+}
+
+// Identification of a device used during its registration.
+message DeviceRegisterIdentification {
+  // The Zero-Touch SN if a device has one.
+  optional string zero_touch_sn = 1;
 }
 
 // Response from server to device register request.