Add EnvelopeGenerator.Service as a .NET Worker Service

Introduced a new project `EnvelopeGenerator.Service` to the solution as a .NET Worker Service targeting the `net4.6.2` framework.

- Configured the project with nullable reference types, implicit usings, and `LangVersion` set to `latest`.
- Added a `Worker` class that logs periodic messages while running.
- Updated `Program.cs` to configure and run the Worker Service.
- Added `launchSettings.json` for development environment configuration.
- Added `appsettings.json` and `appsettings.Development.json` for logging configuration.
- Included the project in `EnvelopeGenerator.sln` with appropriate build configurations and hierarchy updates.

EnvelopeGenerator.Service/Program.cs

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