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/appsettings.json

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