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/rekor-bundle.json

@@ -0,0 +1,6 @@
+{
+  "kind": "rekor.bundle",
+  "apiVersion": "0.1.0",
+  "logIndex": 123456,
+  "payloadHash": "stub"
+}