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/RequestHandler.cs

@@ -0,0 +1,24 @@
+using HrynCo.NotificationService.DAL.Abstract;
+using HrynCo.NotificationService.Services.Logging;
+using MediatR;
+using Serilog;
+
+namespace HrynCo.NotificationService.Services.Core;
+
+public abstract class RequestHandler<TRequest, TResponse> : IRequestHandler<TRequest, TResponse>
+    where TRequest : IRequest<TResponse>
+{
+    protected RequestHandler(IContextualSerilogLogger<TRequest> logger, IUnitOfWork unitOfWork)
+    {
+        Logger = logger.Logger;
+        UnitOfWork = unitOfWork;
+    }
+
+    protected ILogger Logger { get; }
+    protected IUnitOfWork UnitOfWork { get; }
+
+    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken) =>
+        DoOnHandle(request, cancellationToken);
+
+    protected abstract Task<TResponse> DoOnHandle(TRequest request, CancellationToken cancellationToken);
+}