ToolTask process start separated to another function (#9649)

Fix for #9404

Context
We currently do not send the cancelation event to tasks being run by ToolTask, as such, many children task take a long time to cancel, and sometimes they finish executing before said cancelation.
This change makes it possible for inherited classes to access the process and check the cancelation event during execution.

Changes Made
Separated the start of the process to another function that is overridable, as well as exposing the process information.

Notes
Issue will only be closed once C++ team has incorporated this change into the CustomBuild task.

src/Utilities/ToolTask.cs

@@ -658,6 +658,19 @@ protected virtual ProcessStartInfo GetProcessStartInfo(
             return startInfo;
         }
 
+        /// <summary>
+        /// We expect tasks to override this method if they need information about the tool process or its process events during task execution.
+        /// Implementation should make sure that the task is started in this method.
+        /// Starts the process during task execution. 
+        /// </summary>
+        /// <param name="proc">Fully populated <see cref="Process"/> instance representing the tool process to be started.</param>
+        /// <returns>A started process. This could be <paramref name="proc"/> or another <see cref="Process"/> instance.</returns>
+        protected virtual Process StartToolProcess(Process proc)
+        {
+            proc.Start();
+            return proc;
+        }
+
         /// <summary>
         /// Writes out a temporary response file and shell-executes the tool requested.  Enables concurrent
         /// logging of the output of the tool.
@@ -714,7 +727,7 @@ protected virtual int ExecuteTool(
                 ExitCode = -1;
 
                 // Start the process
-                proc.Start();
+                proc = StartToolProcess(proc);
 
                 // Close the input stream. This is done to prevent commands from
                 // blocking the build waiting for input from the user.