add(conversation):完善私聊好友会话服务

backend/IM_API/Tools/SignalRMethodDefine.cs

@@ -0,0 +1,7 @@
+namespace IM_API.Tools
+{
+    public class SignalRMethodDefine
+    {
+        public static string ReceiveMessage = "ReceiveMessage";
+    }
+}

backend/IM_API/Tools/StreamKeyBuilder.cs

@@ -0,0 +1,17 @@
+namespace IM_API.Tools
+{
+    public static class StreamKeyBuilder
+    {
+        public static string Private(long a, long b)
+        {
+            if (a <= 0 || b <= 0) throw new ArgumentException();
+            return $"p:{(a < b ? $"{a}_{b}" : $"{b}_{a}")}";
+        }
+
+        public static string Group(long groupId)
+        {
+            if (groupId <= 0) throw new ArgumentException();
+            return $"g:{groupId}";
+        }
+    }
+}