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/Common/Options/EmailAccountsOptions.cs

@@ -0,0 +1,17 @@
+using DigitalData.MessagingService.Application.Common.Dtos;
+
+namespace DigitalData.MessagingService.Application.Common.Options;
+
+/// <summary>
+/// Wrapper options class that holds a list of <see cref="EmailAccountDto"/> entries
+/// bound from the <c>EmailAccounts</c> configuration section.
+/// </summary>
+public class EmailAccountsOptions
+{
+    public const string SectionName = "EmailAccounts";
+
+    /// <summary>
+    /// The list of configured email accounts.
+    /// </summary>
+    public required IEnumerable<EmailAccountDto> Accounts { get; init; } = [];
+}