Add MongoDB storage library and update acceptance tests with deterministic stubs

- Created StellaOps.Notify.Storage.Mongo project with initial configuration.
- Added expected output files for acceptance tests (at1.txt to at10.txt).
- Added fixture input files for acceptance tests (at1 to at10).
- Created input and signature files for test cases fc1 to fc5.

docs/scripts/sbom-vex/envelope.dsse

@@ -0,0 +1,10 @@
+{
+  "payloadType": "application/vnd.cyclonedx+json",
+  "payload": "ewogICJib21Gb3JtYXQiOiAiQ3ljbG9uZURYIiwKICAic3BlY1ZlcnNpb24iOiAiMS41IiwKICAidmVyc2lvbiI6IDEsCiAgImNvbXBvbmVudHMiOiBbCiAgICB7InR5cGUiOiAiY29udGFpbmVyIiwgIm5hbWUiOiAiZXhhbXBsZSIsICJ2ZXJzaW9uIjogIjEuMC4wIn0KICBdCn0K",
+  "signatures": [
+    {
+      "keyid": "stub-key-id",
+      "sig": "stub-signature"
+    }
+  ]
+}