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/ServiceResult.cs

@@ -0,0 +1,17 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public record ServiceResult<TResult>
+{
+    public ServiceError? Error { get; private set; }
+    public bool IsSuccess { get; private set; }
+    public TResult? Result { get; private set; }
+
+    public static ServiceResult<TResult> Success(TResult result) =>
+        new() { IsSuccess = true, Result = result };
+
+    public static ServiceResult<TResult> Failure(ServiceError error) =>
+        new() { IsSuccess = false, Error = error };
+
+    public static ServiceResult<TResult> Failure(string message, ServiceErrorCode? code = null) =>
+        Failure(new ServiceError(message, code));
+}