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/Runner.Worker/Handlers/CompositeActionHandler.cs

@@ -0,0 +1,98 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using GitHub.DistributedTask.ObjectTemplating.Tokens;
+using GitHub.DistributedTask.Pipelines.ContextData;
+using GitHub.DistributedTask.WebApi;
+using GitHub.Runner.Common;
+using GitHub.Runner.Sdk;
+using Pipelines = GitHub.DistributedTask.Pipelines;
+
+
+namespace GitHub.Runner.Worker.Handlers
+{
+    [ServiceLocator(Default = typeof(CompositeActionHandler))]
+    public interface ICompositeActionHandler : IHandler
+    {
+        CompositeActionExecutionData Data { get; set; }
+    }
+    public sealed class CompositeActionHandler : Handler, ICompositeActionHandler
+    {
+        public CompositeActionExecutionData Data { get; set; }
+
+        public Task RunAsync(ActionRunStage stage)
+        {
+            // Validate args.
+            Trace.Entering();
+            ArgUtil.NotNull(ExecutionContext, nameof(ExecutionContext));
+            ArgUtil.NotNull(Inputs, nameof(Inputs));
+
+            var githubContext = ExecutionContext.ExpressionValues["github"] as GitHubContext;
+            ArgUtil.NotNull(githubContext, nameof(githubContext));
+
+            var tempDirectory = HostContext.GetDirectory(WellKnownDirectory.Temp);
+
+            // Resolve action steps
+            var actionSteps = Data.Steps;
+
+            // Create Context Data to reuse for each composite action step
+            var inputsData = new DictionaryContextData();
+            foreach (var i in Inputs)
+            {
+                inputsData[i.Key] = new StringContextData(i.Value);
+            }
+
+            // Add each composite action step to the front of the queue
+            int location = 0;
+            foreach (Pipelines.ActionStep aStep in actionSteps)
+            {
+                // Ex: 
+                // runs:
+                //      using: "composite"
+                //      steps:
+                //          - uses: example/test-composite@v2 (a)
+                //          - run echo hello world (b)
+                //          - run echo hello world 2 (c)
+                // 
+                // ethanchewy/test-composite/action.yaml
+                // runs:
+                //      using: "composite"
+                //      steps: 
+                //          - run echo hello world 3 (d)
+                //          - run echo hello world 4 (e)
+                // 
+                // Steps processed as follow:
+                // | a |
+                // | a | => | d |
+                // (Run step d)
+                // | a | 
+                // | a | => | e |
+                // (Run step e)
+                // | a | 
+                // (Run step a)
+                // | b | 
+                // (Run step b)
+                // | c |
+                // (Run step c)
+                // Done.
+
+                var actionRunner = HostContext.CreateService<IActionRunner>();
+                actionRunner.Action = aStep;
+                actionRunner.Stage = stage;
+                actionRunner.Condition = aStep.Condition;
+                actionRunner.DisplayName = aStep.DisplayName;
+                // TODO: Do we need to add any context data from the job message?
+                // (See JobExtension.cs ~line 236)
+
+                ExecutionContext.RegisterNestedStep(actionRunner, inputsData, location);
+                location++;
+            }
+
+            return Task.CompletedTask;
+        }
+
+    }
+}