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.

src/AirGap/StellaOps.AirGap.Importer/Models/BundleCatalogEntry.cs

@@ -0,0 +1,8 @@
+namespace StellaOps.AirGap.Importer.Models;
+
+public sealed record BundleCatalogEntry(
+    string TenantId,
+    string BundleId,
+    string Digest,
+    DateTimeOffset ImportedAtUtc,
+    IReadOnlyList<string> ContentPaths);

src/AirGap/StellaOps.AirGap.Importer/Models/BundleItem.cs

@@ -0,0 +1,8 @@
+namespace StellaOps.AirGap.Importer.Models;
+
+public sealed record BundleItem(
+    string TenantId,
+    string BundleId,
+    string Path,
+    string Digest,
+    long SizeBytes);