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/excititor/chunk-attestation-sample.json

@@ -0,0 +1,18 @@
+{
+  "subject_digest": "sha256:112233",
+  "predicates": {
+    "stellaops.vex.chunk.meta.v1": {
+      "tenant": "acme",
+      "source": "ghsa",
+      "schema": "stellaops.vex.chunk.v1",
+      "item_count": 1
+    },
+    "stellaops.vex.chunk.integrity.v1": {
+      "items": [
+        {"ordinal": 0, "sha256": "abc"}
+      ]
+    }
+  },
+  "signing_profile": "sovereign-default",
+  "transparency": null
+}