Add DMTF mockup fixtures and test helpers

Unmodified files from DMTF Redfish-Publications (public-rackmount1, public-localstorage, public-telemetry), only those the tests use, each set with DMTF's BSD-3-Clause license and a COMMANDS.txt naming the source commit and request paths.

conftest.py loads them by request path and serves a mockup over mocked HTTP, with a SessionService that hands out a token, for the transport tests.

tests/fixtures/dmtf-public-rackmount1/UpdateService/FirmwareInventory/SS/index.json

@@ -0,0 +1,34 @@
+{
+    "@odata.type": "#SoftwareInventory.v1_15_0.SoftwareInventory",
+    "Id": "SS",
+    "Name": "Contoso Simple Storage Firmware",
+    "Status": {
+        "State": "Enabled",
+        "Health": "OK"
+    },
+    "Updateable": true,
+    "Manufacturer": "Contoso",
+    "ReleaseDate": "2021-10-18T12:00:00Z",
+    "Version": "2.50",
+    "LowestSupportedVersion": "2.30.rev1",
+    "RelatedItem": [
+        {
+            "@odata.id": "/redfish/v1/Systems/437XR1138R2/SimpleStorage/1"
+        }
+    ],
+    "ImageState": "Active",
+    "Active": true,
+    "Staged": false,
+    "Armed": false,
+    "Links": {
+        "ActiveTargets": [
+            {
+                "@odata.id": "/redfish/v1/Systems/437XR1138R2/SimpleStorage/1"
+            }
+        ],
+        "StagedTargets": [],
+        "ArmedTargets": []
+    },
+    "@odata.id": "/redfish/v1/UpdateService/FirmwareInventory/SS",
+    "@Redfish.Copyright": "Copyright 2014-2026 DMTF. For the full DMTF copyright policy, see http://www.dmtf.org/about/policies/copyright."
+}