Add solution-wide annotation references as non-transitive assets

Directory.build.props

@@ -0,0 +1,6 @@
+<Project>
+  <ItemGroup>
+    <PackageReference Include="JetBrains.Annotations" Version="2023.3.0" PrivateAssets="all" />
+    <PackageReference Include="JetBrains.ExternalAnnotations" Version="10.2.147" PrivateAssets="all" />
+  </ItemGroup>
+</Project>