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/workflow-v1.0.json

@@ -71,7 +71,8 @@
           "env": "job-env",
           "outputs": "job-outputs",
           "defaults": "job-defaults",
-          "steps": "steps"
+          "steps": "steps",
+          "snapshot": "snapshot"
         }
       }
     },
@@ -155,6 +156,24 @@
       }
     },
 
+    "snapshot": {
+      "one-of": [
+        "non-empty-string",
+        "snapshot-mapping"
+      ]
+    },
+
+    "snapshot-mapping": {
+      "mapping": {
+        "properties": {
+          "image-name": {
+            "type": "non-empty-string",
+            "required": true
+          }
+        }
+      }
+    },
+
     "runs-on": {
       "context": [
         "github",