NullReferenceException thrown by Kubernetes.InitializeFromConfig when initialized with HttpClient

[BrandonSharp]

When creating a new client instance by passing in an HttpClient, a NullReferenceException is thrown by the InitializeFromConfig method because the HttpClientHandler property is null. This can be demonstrated by attempting to run the httpClientFactory sample in the examples folder.

Full stack trace:

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at k8s.Kubernetes.InitializeFromConfig(KubernetesClientConfiguration config) in C:\dev\github\third_party\csharp\src\KubernetesClient\Kubernetes.ConfigInit.cs:line 108
   at k8s.Kubernetes..ctor(KubernetesClientConfiguration config, HttpClient httpClient, Boolean disposeHttpClient) in C:\dev\github\third_party\csharp\src\KubernetesClient\Kubernetes.ConfigInit.cs:line 46        at k8s.Kubernetes..ctor(KubernetesClientConfiguration config, HttpClient httpClient) in C:\dev\github\third_party\csharp\src\KubernetesClient\Kubernetes.ConfigInit.cs:line 25
   at httpClientFactory.Program.<>c.<Main>b__0_2(HttpClient httpClient, IServiceProvider serviceProvider) in C:\dev\github\third_party\csharp\examples\httpClientFactory\Program.cs:line 31
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitTransient(TransientCallSite transientCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitTransient(TransientCallSite transientCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService[T](IServiceProvider provider)   at Microsoft.Extensions.Hosting.Internal.Host.StartAsync(CancellationToken cancellationToken)
   at httpClientFactory.Program.Main(String[] args) in C:\dev\github\third_party\csharp\examples\httpClientFactory\Program.cs:line 41
   at httpClientFactory.Program.<Main>(String[] args)

This is likely related to #307, where it was suggested that a fix to the AutoRest library might be necessary. After digging a bit deeper this afternoon, I think this may be an issue that does need to be worked through in the k8s client code. When using .NET Core's HttpClientFactory pattern, by the time you get an HttpClient instance, it's already been initialized with its HttpClientHandler, and there's no (good) way for AutoRest to get this back out. I believe that's the reason the code linked in #307 for ServiceClient doesn't already set the HttpClientHandler property when passed an already existing HttpClient.

The only two solutions I can think of are to either pass an already-configured HttpClient into the Kubernetes client constructor and not run the offending pieces of InitializeFromConfig in that scenario, or pass in both the HttpClient and whatever HttpClientHandler was used to create it. The latter feels a bit wonky, so I've worked up a solution that implements the former, since it plays nicer with the HttpClientFactory implementation that .NET Core provides. Once I've had time to vet a few additional scenarios (i.e., testing with frameworks other than .NET Core), I'll submit my changes as a PR for review.

[BrandonSharp]

Should be fixed by #336