NullReferenceException when GetService<IHttpClientFactory>() fails

[plutora-steve-g]

In ExecuteHttpProxyOperationAsync the following code throws if IHttpClientFactory is not registered. Wasn't obvious to me what the cause was, ended up downloading the code and stepping through

                var httpClient = context.RequestServices
                    .GetService<IHttpClientFactory>()
                    .CreateClient(options?.HttpClientName ?? Helpers.HttpProxyClientName);

Perhaps something along these lines:

                var clientFactory = context.RequestServices.GetService<IHttpClientFactory>()
                     ?? throw new InvalidOperationException("IHttpClientFactory not registered. You need to add builder.Services.AddHttpClient(); during startup");
                var httpClient = clientFactory.CreateClient(options?.HttpClientName ?? Helpers.HttpProxyClientName);

[twitchax]

Good idea: if you want to make the PR, then you can get credit. 😄

Otherwise, I am happy to do it.