The `SendAsync` method in the `MessagingServiceClient` class has been refactored. It no longer uses the `async` keyword and instead directly returns the `Task` from `sender.Send`. This change simplifies the method by removing the `await` operation, while still maintaining asynchronous behavior through the returned `Task`.
Introduce the Email contract and MessagingServiceClient that send mail through the Application layer via MediatR, and add AddMessagingServiceClient overloads for delegate- and IConfiguration-based setup. Remove the empty client DI placeholders.
