Add new features and tests for AirGap and Time modules

- Introduced `SbomService` tasks documentation.
- Updated `StellaOps.sln` to include new projects: `StellaOps.AirGap.Time` and `StellaOps.AirGap.Importer`.
- Added unit tests for `BundleImportPlanner`, `DsseVerifier`, `ImportValidator`, and other components in the `StellaOps.AirGap.Importer.Tests` namespace.
- Implemented `InMemoryBundleRepositories` for testing bundle catalog and item repositories.
- Created `MerkleRootCalculator`, `RootRotationPolicy`, and `TufMetadataValidator` tests.
- Developed `StalenessCalculator` and `TimeAnchorLoader` tests in the `StellaOps.AirGap.Time.Tests` namespace.
- Added `fetch-sbomservice-deps.sh` script for offline dependency fetching.

docs/samples/evidence-locker/attestation-v1-sample.json

@@ -0,0 +1,18 @@
+{
+  "subject_digest": "sha256:deadbeef",
+  "bundle_id": "11111111-2222-3333-4444-555555555555",
+  "produced_at": "2025-11-20T00:00:00Z",
+  "producer": "evidence-locker:us-gov-west",
+  "hashes": {
+    "sboms/spdx.json": "abcdef",
+    "vex/osv.json": "123456"
+  },
+  "sbom": [
+    {"digest": "sha256:abcdef", "mediaType": "application/spdx+json"}
+  ],
+  "vex": [
+    {"digest": "sha256:123456", "schema": "openvex-1.0"}
+  ],
+  "signing_profile": "sovereign-default",
+  "transparency": null
+}