Refactor configuration handling

Refactored `MessagingServiceApplicationConfiguration` to simplify the tracking of the LuckyPennySoft license key configuration by replacing the `_configured` tuple with a boolean. Removed email account configuration methods from this class.

Updated `MessagingServiceInfrastructureConfiguration` to introduce a new `_configured` tuple for tracking `RabbitMqConnectionFactory` and `EmailAccounts` configurations. Added methods for configuring email accounts and updated the `EnsureRabbitMqConnectionFactoryAdded` method to include email account checks.

Changed the namespace of `EmailAccountsOptions.cs` to `DigitalData.MessagingService.Infrastructure.Options`.

Modified `Program.cs` to include email account configuration in the infrastructure setup.

Added new files `DependencyInjectionExtensions.cs` and `MessagingServiceConfiguration.cs` as placeholders for future extensions, wrapped in `#if NET` directives.

Removed an `<ItemGroup>` from `DigitalData.MessagingService.Infrastructure.csproj` and updated `DigitalData.MessagingService.Client.csproj` for XML compliance.

src/infrastructure/DigitalData.MessagingService.Infrastructure/Options/EmailAccountsOptions.cs

@@ -0,0 +1,31 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Dto.EmailAccounts;
+
+namespace DigitalData.MessagingService.Infrastructure.Options;
+
+/// <summary>
+/// Wrapper options class that holds a list of <see cref="Dto.EmailAccount"/> 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 List<EmailAccountModificationDto> Accounts { get; set; } = [];
+
+    /// <summary>
+    /// How often the IMAP sync worker polls for new emails, in seconds.
+    /// Defaults to 300 seconds (5 minutes).
+    /// </summary>
+    public int SyncIntervalSeconds { get; set; } = 300;
+
+    /// <summary>
+    /// The minimum interval, in seconds, between forced IMAP sync operations regardless of idle state.
+    /// Defaults to 30 seconds (0.5 minute).
+    /// </summary>
+    public int ForcedSyncIntervalSeconds { get; set; } = 30;
+}
+#endif