Prevent NullReferenceException when using HttpClient to initialize Kubernetes client. Fixes #335

[BrandonSharp]

This should correct the NullReferenceException that occurs when passing in an HttpClient to the constructor of the Kubernetes client. This occurred because HttpClient objects are already initialized with an HttpClientHandler before being handed down to the Kubernetes client, and there's no way to retrieve that object from an HttpClient once initialized.

Because it's useful to set some values on an HttpClientHandler based on the Kubernetes configuration, I added a new file to the partial KubernetesClientConfiguration class that handles generation of a default HttpClientHandler based on the config. This replaced the need to call InitializeFromConfig in the Kubernetes.ConfigInit.cs class from the constructor that accepts an HttpClient. I've also updated the example named httpClientFactory to illustrate the use of this new function.

I also split the SetCredentials method in Kubernetes.ConfgInit.cs into two separate functions. Originally, this method was both setting credentials, and adding certificates to the HttpClientHandler. Because of the changes required to resolve this bug, it was necessary to pull the certificate related code out, and make it more generally usable, given an HttpClientHandler and a config object.

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[BrandonSharp]

Rebased my fork to pick up the recent changes in master that seems to have addressed some of the failing tests.

[brendandburns]

Sorry for the delay here, holidays got in the way, I will try to review in the next day or so.

Thanks for the patience.

[brendandburns]

/lgtm
/approve

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This pull-request has been approved by: BrandonSharp, brendandburns

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