Composite Actions Support for Multiple Run Steps (#549)

* Composite Action Run Steps

* Clean up trace messages + add Trace debug in ActionManager

* Change String to string

* Add comma to Composite

* Change JobSteps to a List, Change Register Step function name

* Add TODO, remove unn. content

* Remove unnecessary code

* Fix unit tests

* Add verbose trace logs which are only viewable by devs

* Sort usings in Composite Action Handler

* Change 0 to location

* Update context variables in composite action yaml

* Add helpful error message for null steps

src/Sdk/DTPipelines/Pipelines/PipelineConstants.cs

@@ -94,5 +94,12 @@ namespace GitHub.DistributedTask.Pipelines
             public static readonly String Resources = "resources";
             public static readonly String All = "all";
         }
+
+        public static class ScriptStepInputs
+        {
+            public static readonly String Script = "script";
+            public static readonly String WorkingDirectory = "workingDirectory";
+            public static readonly String Shell = "shell";
+        }
     }
 }