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/EmailAccount/Queries/GetSenderQuery.cs

@@ -1,4 +1,5 @@
-using DigitalData.MessagingService.Application.Common.Options;
+#if NET
+using DigitalData.MessagingService.Application.Common.Options;
 using DigitalData.MessagingService.Abstraction;
 using MediatR;
 using Microsoft.Extensions.Logging;
@@ -22,7 +23,7 @@ public class GetSenderQueryHandler(IOptions<EmailAccountsOptions> Options, ILogg
 {
     public Task<EmailAccountDto?> Handle(GetSenderQuery request, CancellationToken cancellationToken)
     {
-        var accounts = request.Id is not null 
+        var accounts = request.Id is not null
             ? Options.Value.Accounts.Where(a => a.Id == request.Id)
             : Options.Value.Accounts.Where(a => a.Username == request.Username);
 
@@ -36,4 +37,5 @@ public class GetSenderQueryHandler(IOptions<EmailAccountsOptions> Options, ILogg
 
         return Task.FromResult(accounts.FirstOrDefault());
     }
-}
+}
+#endif