Add example of full-page prerendering via a custom action result

samples/misc/Webpack/ActionResults/PrerenderResult.cs

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+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Extensions;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.NodeServices;
+using Microsoft.AspNetCore.SpaServices.Prerendering;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Webpack.ActionResults
+{
+    // This is an example of how you could invoke the prerendering API from an ActionResult, so as to
+    // prerender a SPA component as the entire response page (instead of injecting the SPA component
+    // into a Razor view's output)
+    public class PrerenderResult : ActionResult
+    {
+        private JavaScriptModuleExport _moduleExport;
+        private object _dataToSupply;
+
+        public PrerenderResult(JavaScriptModuleExport moduleExport, object dataToSupply = null)
+        {
+            _moduleExport = moduleExport;
+            _dataToSupply = dataToSupply;
+        }
+
+        public override async Task ExecuteResultAsync(ActionContext context)
+        {
+            var nodeServices = context.HttpContext.RequestServices.GetRequiredService<INodeServices>();
+            var hostEnv = context.HttpContext.RequestServices.GetRequiredService<IHostingEnvironment>();
+            var applicationBasePath = hostEnv.ContentRootPath;
+            var request = context.HttpContext.Request;
+            var response = context.HttpContext.Response;
+
+            var prerenderedHtml = await Prerenderer.RenderToString(
+                applicationBasePath,
+                nodeServices,
+                _moduleExport,
+                request.GetEncodedUrl(),
+                request.Path + request.QueryString.Value,
+                _dataToSupply
+            );
+
+            response.ContentType = "text/html";
+            await response.WriteAsync(prerenderedHtml.Html);
+        }
+    }
+}

samples/misc/Webpack/ActionResults/PrerenderResultExtensions.cs

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+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.SpaServices.Prerendering;
+
+namespace Webpack.ActionResults
+{
+    public static class PrerenderResultExtensions
+    {
+        public static PrerenderResult Prerender(this ControllerBase controller, JavaScriptModuleExport exportToPrerender, object dataToSupply = null)
+        {
+            return new PrerenderResult(exportToPrerender, dataToSupply);
+        }
+    }
+}