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/core/DigitalData.MessagingService.Application/DependencyInjection.cs

@@ -1,7 +1,9 @@
-using System.Reflection;
+using DigitalData.MessagingService.Application.Common.Options;
 using FluentValidation;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
+using System.Reflection;
 
 namespace DigitalData.MessagingService.Application;
 
@@ -35,6 +37,9 @@ public static class DependencyInjection
         // FluentValidation - Register all validators
         services.AddValidatorsFromAssembly(assembly);
 
+        // Register EmailAccounts configuration (IOptions<EmailAccountsOptions>)
+        services.Configure<EmailAccountsOptions>(configuration.GetSection(EmailAccountsOptions.SectionName));
+
         return services;
     }
 }