Refactor email account configuration for multi-account support

Refactored `EmailAccountDto` to represent a single account with
immutable properties and added an `Id` field. Introduced
`EmailAccountsOptions` to manage multiple accounts and bound it
to the `EmailAccounts` configuration section.

Updated `DependencyInjection` to register `EmailAccountsOptions`
and removed the old single-account binding. Refactored
`LimilabsEmailService` to use `EmailAccountsOptions` and select
the appropriate account dynamically.

Replaced the `EmailAccount` section in `appsettings.Secrets.json`
with a new `EmailAccounts` section supporting multiple accounts.
Added a package reference for `Microsoft.Extensions.Options.
ConfigurationExtensions` to support the options pattern.

src/presentation/DigitalData.MessagingService.API/Program.cs

@@ -67,10 +67,6 @@ try
     // Register Infrastructure layer (RabbitMQ, Repositories, etc.)
     builder.Services.AddInfrastructure(builder.Configuration);
 
-    // Register EmailAccount configuration (IOptions<EmailAccountDto>)
-    builder.Services.Configure<EmailAccountDto>(
-        builder.Configuration.GetSection("EmailAccount"));
-
     builder.Services.AddControllers();
 
     builder.Services.AddEndpointsApiExplorer();