feat: add MediatR handlers for template and channel CRUD

- ServiceResult<T>, ServiceError, ServiceErrorCode, Unit, ServiceResultHelper in Services/Core
- RequestHandler<TRequest, TResponse> base class (MediatR-adapted, DoOnHandle pattern)
- EmailTemplate handlers: Create, Update, Delete, Get, GetByService
- EmailChannel handlers: Create, Update, Delete, Get, GetByService

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Services/Core/ServiceError.cs

@@ -0,0 +1,13 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public sealed record ServiceError
+{
+    public ServiceError(string message, ServiceErrorCode? code = null)
+    {
+        Message = message;
+        Code = code;
+    }
+
+    public ServiceErrorCode? Code { get; set; }
+    public string Message { get; set; }
+}