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/appsettings.Development.json

@@ -0,0 +1,8 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.Hosting.Lifetime": "Information"
+    }
+  }
+}