Expose email addresses for ExternalUsers

This change allows instances that make use of ExternalUser to
also associate email addresses with ExternalUsers. Those can
be used to compute group memberships in external group backends
or associate the user with Git commits.

This commit is a follow-up the series in I2d87e7bb.

Change-Id: I2e0a598c84d03113c8ea078b1148c5c598e1d495

javatests/com/google/gerrit/acceptance/server/permissions/ExternalUserPermissionIT.java

@@ -293,6 +293,7 @@ public class ExternalUserPermissionIT extends AbstractDaemonTest {
 
   ExternalUser createUserInGroup(String userId, String groupId) {
     return externalUserFactory.create(
+        ImmutableSet.of(),
         ImmutableSet.of(ExternalId.Key.parse("company-auth:" + groupId + "-" + userId)));
   }
 }