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/Common/Options/EmailAccountsOptions.cs

@@ -1,4 +1,5 @@
-using DigitalData.MessagingService.Abstraction;
+#if NET
+using DigitalData.MessagingService.Abstraction;
 
 namespace DigitalData.MessagingService.Application.Common.Options;
 
@@ -21,3 +22,4 @@ public class EmailAccountsOptions
     /// </summary>
     public int SyncIntervalSeconds { get; init; } = 300;
 }
+#endif