Add multi-targeting support for .NET frameworks

Introduced conditional compilation using `#if NET` directives to
enable support for multiple frameworks (`net462`, `net480`, and
`net8.0`). Updated the project file to support multi-targeting
and added framework-specific dependencies conditionally.

Refactored namespaces, interfaces, classes, validators, and
handlers to ensure compatibility with targeted frameworks.
Enhanced dependency injection setup and adjusted logic in
`GetSenderQueryHandler` for framework-specific behavior.

Ensured consistency and maintainability by wrapping framework-
specific code blocks with `#if NET` directives.

src/core/DigitalData.MessagingService.Application/DependencyInjection.cs

@@ -1,8 +1,8 @@
+#if NET
 using DigitalData.MessagingService.Application.Common.Options;
 using FluentValidation;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.Options;
 using System.Reflection;
 
 namespace DigitalData.MessagingService.Application;
@@ -43,3 +43,4 @@ public static class DependencyInjection
         return services;
     }
 }
+#endif