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/Interfaces/IImapEmailService.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Abstraction;
 using DigitalData.MessagingService.Application.Common.Models.MailSearch;
 
@@ -31,3 +32,4 @@ public interface IImapEmailService
         string folder = "INBOX",
         CancellationToken cancellationToken = default);
 }
+#endif