DbContext class separated.

code/MessengerBroker.Db.Sql/BrokerSqlDbContext.cs

@@ -0,0 +1,20 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace MessengerBroker.Db.Sql
+{
+    public class BrokerSqlDbContext : BrokerDbContext
+    {
+        private readonly string connectionString;
+
+        public BrokerSqlDbContext(string connectionString)
+        {
+            this.connectionString = connectionString;
+        }
+
+        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+        {
+            base.OnConfiguring(optionsBuilder);
+            optionsBuilder.UseSqlServer(this.connectionString);
+        }
+    }
+}