add IsolationLevel for BeginTranAsync

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

@@ -223,6 +223,12 @@ namespace SqlSugar
             if (this.Transaction == null)
                 this.Transaction = this.Connection.BeginTransaction(iso);
         }
+        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 RollbackTran()
         {
             if (this.Transaction != null)

Src/Asp.Net/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.Net/SqlSugar/OnlyNet/AsyncHelper.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Data;
 using System.Data.Common;
 using System.Linq;
 using System.Text;
@@ -15,6 +16,14 @@ namespace SqlSugar
             //.net frame work no BeginTransactionAsync
             return db.BeginTransaction();
         }
+        
+        public static async Task<DbTransaction> BeginTransactionAsync(this DbConnection db, IsolationLevel iso)
+        {
+            await Task.Delay(0);
+            //.net frame work no BeginTransactionAsync
+            return db.BeginTransaction(iso);
+        }
+        
         public static async Task CloseAsync(this DbConnection db)
         {
             await Task.Delay(0);

Src/Asp.Net/SqlSugar/SqlSugarClient.cs

@@ -929,6 +929,12 @@ namespace SqlSugar
             _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()
         {