Add constructor overload to .NET DriverServiceCommandExecutor

This new constructor overload allows the user to supply their own
HttpCommandExecutor (or a subclass) for local execution where the
driver service executable exists locally, and the user wants the
driver instance to own the process life cycle of the executable.

dotnet/src/webdriver/Remote/DriverServiceCommandExecutor.cs

@@ -52,6 +52,18 @@ public DriverServiceCommandExecutor(DriverService driverService, TimeSpan comman
             this.internalExecutor = new HttpCommandExecutor(driverService.ServiceUrl, commandTimeout, enableKeepAlive);
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DriverServiceCommandExecutor"/> class.
+        /// </summary>
+        /// <param name="driverService">The <see cref="DriverService"/> that drives the browser.</param>
+        /// <param name="commandExecutor">The <see cref="HttpCommandExecutor"/> object used to execute commands,
+        /// communicating with the service via HTTP.</param>
+        public DriverServiceCommandExecutor(DriverService service, HttpCommandExecutor commandExecutor)
+        {
+            this.service = service;
+            this.internalExecutor = commandExecutor;
+        }
+
         /// <summary>
         /// Gets the <see cref="CommandInfoRepository"/> object associated with this executor.
         /// </summary>