Process snapshot tokens (#3135)

* Added Snapshot TemplateToken to AgentJobRequestMessage

* WIP for processing the snapshot token

* Changed snapshot post job step condition to Success, added comments

* Refactored snapshot post-job step

* Added evaluation of snapshot token to retrieve image name

* Added snapshot to workflow schema

* Fixed linter error

* Migrated snapshot logic to new SnapshotOperationProvider

* Fixed linter error

* Fixed linter errors

* Fixed linter error

* Fixed linter errors

* Updated L0 tests

* Fixed linter errors

* Added new JobExtensionL0 tests for snapshot post-job step

* Added JobExtensionL0 test case for snapshot mappings

* Added SnapshotOperationProviderL0 tests

* Enabled nullable types for SnapshotOperationProvider and its tests

* Added more assertions to SnapshotOperationProviderL0 tests

* Fixed linter errors

* Made sure TestHostContexts are disposed of properlyh in SnapshotOperationProviderL0 tests

* Resolved PR comments

* Fixed formatting

* Removed redundant reference

* Addressed PR comments

src/Sdk/DTPipelines/Pipelines/Snapshot.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.DistributedTask.Pipelines
+{
+    [DataContract]
+    public class Snapshot
+    {
+        public Snapshot(string imageName)
+        {
+            ImageName = imageName;
+        }
+
+        [DataMember(EmitDefaultValue = false)]
+        public String ImageName { get; set; }
+    }
+}