Add EnvelopeGenerator.WorkerService as background worker

Added a new WorkerService project to the solution, targeting .NET 8.0 and using Microsoft.Extensions.Hosting. Implemented a Worker class that logs periodically, set up hosting in Program.cs, and included configuration files for logging and development. Updated the solution file to include and configure the new project.

EnvelopeGenerator.WorkerService/Program.cs

@@ -0,0 +1,7 @@
+using EnvelopeGenerator.WorkerService;
+
+var builder = Host.CreateApplicationBuilder(args);
+builder.Services.AddHostedService<Worker>();
+
+var host = builder.Build();
+host.Run();