add ability to get user based on username or email address

Oqtane.Client/Services/Interfaces/IUserService.cs

@@ -15,7 +15,6 @@ namespace Oqtane.Services
         /// <param name="siteId">ID of a <see cref="Site"/></param>
         /// <returns></returns>
         Task<User> GetUserAsync(int userId, int siteId);
-
         
         /// <summary>
         /// Get a <see cref="User"/> of a specific site
@@ -25,6 +24,15 @@ namespace Oqtane.Services
         /// <returns></returns>
         Task<User> GetUserAsync(string username, int siteId);
 
+        /// <summary>
+        /// Get a <see cref="User"/> of a specific site
+        /// </summary>
+        /// <param name="username">Username / login of a <see cref="User"/></param>
+        /// <param name="email">email address of a <see cref="User"/></param>
+        /// <param name="siteId">ID of a <see cref="Site"/></param>
+        /// <returns></returns>
+        Task<User> GetUserAsync(string username, string email, int siteId);
+
         /// <summary>
         /// Save a user to the Database.
         /// The <see cref="User"/> object contains all the information incl. what <see cref="Site"/> it belongs to.