refactor: 去掉流程实例的ActivityType，重构节点类型相关代码

OpenAuth.Repository/Domain/FlowInstance.cs

@@ -70,11 +70,6 @@ namespace OpenAuth.Repository.Domain
     [Description("当前节点ID")]
     public string ActivityId { get; set; }
     /// <summary>
-    /// 当前节点类型（0会签节点）
-    /// </summary>
-    [Description("当前节点类型（0会签节点）")]
-    public int? ActivityType { get; set; }
-    /// <summary>
     /// 当前节点名称
     /// </summary>
     [Description("当前节点名称")]

OpenAuth.Repository/Domain/FlowInstanceTransitionHistory.cs

@@ -50,7 +50,7 @@ namespace OpenAuth.Repository.Domain
     /// 开始节点类型
     /// </summary>
     [Description("开始节点类型")]
-    public int? FromNodeType { get; set; }
+    public string FromNodeType { get; set; }
     /// <summary>
     /// 开始节点名称
     /// </summary>
@@ -65,7 +65,7 @@ namespace OpenAuth.Repository.Domain
     /// 结束节点类型
     /// </summary>
     [Description("结束节点类型")]
-    public int? ToNodeType { get; set; }
+    public string ToNodeType { get; set; }
     /// <summary>
     /// 结束节点名称
     /// </summary>