Refactor namespace and project structure

Replaced `DigitalData.MessagingService.Client.DependencyInjection`
namespace with `DigitalData.MessagingService.Client` across the
codebase to simplify and streamline the project structure.

Removed unused `Microsoft.Extensions.Logging` and `System`
dependencies from `EmailSender.cs`. Updated project references
in `DigitalData.MessagingService.Tests.csproj` to reflect the
namespace and project restructuring. Adjusted `using` directives
in test files to align with the new namespace.

src/presentation/DigitalData.MessagingService.Client/OnReconnect.cs

@@ -0,0 +1,20 @@
+namespace DigitalData.MessagingService.Client;
+
+/// <summary>
+/// Defines the behavior when <see cref="EmailSender.ConnectRabbitMq"/> is called
+/// while a connection has already been established.
+/// </summary>
+public enum OnReconnect
+{
+    /// <summary>
+    /// Throws an <see cref="System.InvalidOperationException"/> if the messaging service
+    /// is already connected. This is the default behavior.
+    /// </summary>
+    ThrowException,
+
+    /// <summary>
+    /// Silently ignores the reconnection attempt if the messaging service
+    /// is already connected.
+    /// </summary>
+    Ignore
+}