Support downloading/publishing artifacts from Pipelines endpoint (#188)

* Support downloading/publishing artifacts from Pipelines endpoint
* Remove `Path` from everywhere
* Remove unused JobId argument
* PR feedback
* More PR feedback

src/Runner.Plugins/Artifact/PipelinesServer.cs

@@ -0,0 +1,60 @@
+using System;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using GitHub.Actions.Pipelines.WebApi;
+using GitHub.Services.WebApi;
+using GitHub.Runner.Sdk;
+using Pipelines = GitHub.Actions.Pipelines.WebApi;
+
+namespace GitHub.Runner.Plugins.Artifact
+{
+    // A client wrapper interacting with Pipelines's Artifact API
+    public class PipelinesServer
+    {
+        private readonly PipelinesHttpClient _pipelinesHttpClient;
+
+        public PipelinesServer(VssConnection connection)
+        {
+            ArgUtil.NotNull(connection, nameof(connection));
+            _pipelinesHttpClient = connection.GetClient<PipelinesHttpClient>();
+        }
+
+        // Associate the specified Actions Storage artifact with a pipeline
+        public async Task<Pipelines.ActionsStorageArtifact> AssociateActionsStorageArtifactAsync(
+            int pipelineId,
+            int runId,
+            long containerId,
+            string name,
+            long size,
+            CancellationToken cancellationToken = default(CancellationToken))
+        {
+            CreateArtifactParameters parameters = new CreateActionsStorageArtifactParameters()
+            {
+                Name = name,
+                ContainerId = containerId,
+                Size = size
+            };
+
+            return await _pipelinesHttpClient.CreateArtifactAsync(
+                parameters,
+                pipelineId, 
+                runId,
+                cancellationToken: cancellationToken) as Pipelines.ActionsStorageArtifact;
+        }
+
+        // Get named Actions Storage artifact for a pipeline
+        public async Task<Pipelines.ActionsStorageArtifact> GetActionsStorageArtifact(
+            int pipelineId,
+            int runId,
+            string name,
+            CancellationToken cancellationToken)
+        {
+            return await _pipelinesHttpClient.GetArtifactAsync(
+                pipelineId,
+                runId,
+                name,
+                cancellationToken: cancellationToken) as Pipelines.ActionsStorageArtifact;
+        }
+    }
+}