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/Concelier/__Libraries/StellaOps.Concelier.Core/Linksets/ObservationPipelineServiceCollectionExtensions.cs

@@ -12,6 +12,7 @@ public static class ObservationPipelineServiceCollectionExtensions
         ArgumentNullException.ThrowIfNull(services);
 
         services.TryAddSingleton<IAdvisoryObservationSink, NullObservationSink>();
+        services.TryAddSingleton<IAdvisoryObservationEventPublisher, NullObservationEventPublisher>();
         services.TryAddSingleton<IAdvisoryLinksetSink, NullLinksetSink>();
         services.TryAddSingleton<IAdvisoryLinksetLookup, NullLinksetLookup>();
         services.TryAddSingleton<IAdvisoryLinksetBackfillService, AdvisoryLinksetBackfillService>();
@@ -25,6 +26,12 @@ public static class ObservationPipelineServiceCollectionExtensions
             => Task.CompletedTask;
     }
 
+    private sealed class NullObservationEventPublisher : IAdvisoryObservationEventPublisher
+    {
+        public Task PublishAsync(AdvisoryObservationUpdatedEvent @event, CancellationToken cancellationToken)
+            => Task.CompletedTask;
+    }
+
     private sealed class NullLinksetSink : IAdvisoryLinksetSink
     {
         public Task UpsertAsync(AdvisoryLinkset linkset, CancellationToken cancellationToken)