Refactor email account handling for dynamic resolution

Reintroduced `EmailAccountDto` with conditional compilation to support both .NET and non-.NET environments. Updated `IEmailService` to accept `EmailAccountDto` as the sender, replacing reliance on pre-configured SMTP credentials.

Added `GetSenderQuery` and its handler to dynamically resolve email accounts based on `Id` or `Username`. Introduced `GetSenderQueryValidator` for validation, ensuring proper usage of the query.

Modified `SendEmailCommand` to include sender resolution via MediatR. Updated `OutgoingEmailEvent` to include sender information and adjusted `OutgoingEmailConsumer` and `LimilabsEmailService` to use the dynamically resolved sender.

Updated `EmailMappingProfile` to ignore the `Sender` property during mapping. Replaced `Name` with `Id` in `appsettings.Secrets.json` for email accounts. Removed the old `EmailAccountDto` folder and performed general cleanup and restructuring.

src/core/DigitalData.MessagingService.Application/EmailAccount/Queries/GetSenderQuery.cs

@@ -0,0 +1,39 @@
+using DigitalData.MessagingService.Application.Common.Dtos;
+using DigitalData.MessagingService.Application.Common.Options;
+using MediatR;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace DigitalData.MessagingService.Application.EmailAccount.Queries;
+
+public record GetSenderQuery : IRequest<EmailAccountDto?>
+{
+    public int? Id { get; init; }
+
+    public string? Username { get; init; }
+}
+
+/// <summary>
+/// 
+/// </summary>
+/// <param name="Options"></param>
+/// <param name="Logger"></param>
+public class GetSenderQueryHandler(IOptions<EmailAccountsOptions> Options, ILogger<GetSenderQueryHandler> Logger) : IRequestHandler<GetSenderQuery, EmailAccountDto?>
+{
+    public Task<EmailAccountDto?> Handle(GetSenderQuery request, CancellationToken cancellationToken)
+    {
+        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);
+
+        if(accounts.Count() > 1)
+        {
+            Logger.LogWarning(
+                "Multiple email accounts found for the given criteria ({Criteria}). Returning the first one.",
+                request.Id is not null ? $"Id: {request.Id}" : $"Username: {request.Username}"
+            );
+        }
+
+        return Task.FromResult(accounts.FirstOrDefault());
+    }
+}