Add EnvelopeGenerator.ServiceHost ASP.NET Core Web API project

Created a new EnvelopeGenerator.ServiceHost project targeting .NET 8.0. Set up minimal API host with controllers, Swagger/OpenAPI support, and development configuration files. Updated solution to include the new project with appropriate build settings.

EnvelopeGenerator.ServiceHost/appsettings.json

@@ -0,0 +1,9 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*"
+}