Initialize minimal ASP.NET Core API with OpenAPI support

Set up a basic ASP.NET Core project targeting .NET 9.0 with nullable and implicit usings enabled. Added OpenAPI (Swagger) documentation using NSwag and a simple WeatherForecast endpoint. Included necessary configuration files for development and toolchain support.

Api/appsettings.Development.json

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