added a GetTokenAsync method that calls the Api method, and transform the raw Api Token to a RestToken.

src/Discord.Net.Rest/ClientHelper.cs

@@ -16,6 +16,11 @@ namespace Discord.Rest
             var model = await client.ApiClient.GetMyApplicationAsync(options).ConfigureAwait(false);
             return RestApplication.Create(client, model);
         }
+        public static async Task<RestToken> GetTokenAsync(BaseDiscordClient client, TokenType tokenType, string token, string redirectUrl, IEnumerable<string> scopes, RequestOptions options)
+        {
+            var model = await client.ApiClient.GetTokenAsync(tokenType, token, redirectUrl, scopes, options).ConfigureAwait(false);
+            return RestToken.Create(client, model);
+        }
 
         public static async Task<RestChannel> GetChannelAsync(BaseDiscordClient client, 
             ulong id, RequestOptions options)