add IsolationLevel for BeginTranAsync

Src/Asp.NetCore2/SqlSugar/Abstract/AdoProvider/AdoProvider.cs

@@ -217,6 +217,14 @@ namespace SqlSugar
             if (this.Transaction == null)
                 this.Transaction =await (this.Connection as DbConnection).BeginTransactionAsync();
         }
+        
+        public virtual async Task BeginTranAsync(IsolationLevel iso)
+        {
+            await CheckConnectionAsync();
+            if (this.Transaction == null)
+                this.Transaction =await (this.Connection as DbConnection).BeginTransactionAsync(iso);
+        }
+        
         public virtual void BeginTran(IsolationLevel iso)
         {
             CheckConnection();

Src/Asp.NetCore2/SqlSugar/Interface/IAdo.cs

@@ -172,6 +172,7 @@ namespace SqlSugar
 
         void BeginTran();
         Task BeginTranAsync();
+        Task BeginTranAsync(IsolationLevel iso);
         void BeginTran(IsolationLevel iso);
         void BeginTran(string transactionName);
         void BeginTran(IsolationLevel iso, string transactionName);

Src/Asp.NetCore2/SqlSugar/SqlSugarClient.cs

@@ -923,12 +923,24 @@ namespace SqlSugar
             _IsAllTran = true;
             AllClientEach(it => it.Ado.BeginTran());
         }
+        
+        public void BeginTran(IsolationLevel iso)
+        {
+            _IsAllTran = true;
+            AllClientEach(it => it.Ado.BeginTran(iso));
+        }
 
         public async Task BeginTranAsync()
         {
             _IsAllTran = true;
             await AllClientEachAsync(async it => await it.Ado.BeginTranAsync());
         }
+        
+        public async Task BeginTranAsync(IsolationLevel iso)
+        {
+            _IsAllTran = true;
+            await AllClientEachAsync(async it => await it.Ado.BeginTranAsync(iso));
+        }
 
         public void CommitTran()
         {