Add generic CreateCommandHandler for create operations

Introduced a generic CreateCommandHandler class implementing MediatR's IRequestHandler to handle create commands. The handler uses a generic IRepository to perform asynchronous entity creation and supports dependency injection for repository access.

EnvelopeGenerator.Application/Common/Commands/CreateCommandHandler.cs

@@ -0,0 +1,36 @@
+using DigitalData.Core.Abstraction.Application.Repository;
+using MediatR;
+
+namespace EnvelopeGenerator.Application.Common.Commands;
+
+/// <summary>
+/// 
+/// </summary>
+/// <typeparam name="TCommand"></typeparam>
+/// <typeparam name="TEntity"></typeparam>
+public class CreateCommandHandler<TCommand, TEntity> : IRequestHandler<TCommand>
+    where TCommand : class, IRequest
+    where TEntity : class
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    protected readonly IRepository<TEntity> Repository;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    public CreateCommandHandler(IRepository<TEntity> repository)
+    {
+        Repository = repository;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancel"></param>
+    /// <returns></returns>
+    public Task Handle(TCommand request, CancellationToken cancel) => Repository.CreateAsync(request, cancel);
+}