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.

.gitea/workflows/obs-stream.yml

@@ -14,6 +14,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Task Pack offline bundle fixtures
+        run: python3 scripts/packs/run-fixtures-check.sh
+
       - name: Install nats CLI
         run: |
           curl -sSL https://github.com/nats-io/natscli/releases/download/v0.1.4/nats-0.1.4-linux-amd64.tar.gz -o /tmp/natscli.tgz