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/Test/L0/Worker/JobRunnerL0.cs

@@ -101,6 +101,7 @@ namespace GitHub.Runner.Common.Tests.Worker
                 testName,
                 testName, null, null, null, new Dictionary<string, VariableValue>(), new List<MaskHint>(), new Pipelines.JobResources(), new Pipelines.ContextData.DictionaryContextData(), new Pipelines.WorkspaceOptions(), new List<Pipelines.ActionStep>(), null, null, null,
                 new ActionsEnvironmentReference("staging"),
+                null,
                 messageType: messageType);
             message.Variables[Constants.Variables.System.Culture] = "en-US";
             message.Resources.Endpoints.Add(new ServiceEndpoint()