[SPA] Set accept header on HttpClient used in SpaProxyLaunchManager (#33135)

This unblocks other JS build tools for working as the frontend proxy.

Author: johanndev

src/Middleware/Spa/SpaProxy/src/SpaProxyLaunchManager.cs

@@ -7,6 +7,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Net.Http;
+using System.Net.Http.Headers;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -37,6 +38,9 @@ public void StartInBackground(CancellationToken cancellationToken)
                 // It's ok for us to do this here since this service is only plugged in during development.
                 ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator
             });
+            // Certain frontend build tools rely on the the ACCEPT Header being set, otherwise these tools
+            // might be unable to perform their client-side fallback logic.
+            httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*"));
             _logger.LogInformation($"No SPA development server running at {_options.ServerUrl} found.");
 
             // We are not waiting for the SPA proxy to launch, instead we are going to rely on a piece of