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/Worker.cs

@@ -0,0 +1,24 @@
+namespace EnvelopeGenerator.Service
+{
+    public class Worker : BackgroundService
+    {
+        private readonly ILogger<Worker> _logger;
+
+        public Worker(ILogger<Worker> logger)
+        {
+            _logger = logger;
+        }
+
+        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+        {
+            while (!stoppingToken.IsCancellationRequested)
+            {
+                if (_logger.IsEnabled(LogLevel.Information))
+                {
+                    _logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now);
+                }
+                await Task.Delay(1000, stoppingToken);
+            }
+        }
+    }
+}