Dispose certificates in Kubernetes.Dispose()

[shenglol]

The Kubernetes constructor creates client and SSL CA certs via new X509Certificate2(...) if the kubeconfig contains cert path/data. As a result, on Windows, some one-time use files may be created for each new Kuberentes(...) call (see The most dangerous constructor in .NET).

According to the SO answer, the files will be removed by GC and calling Dispose is not required. However, since GC is not guaranteed to invoke all finalizers, we cannot rely on it to do the cleanup. I ran a local test and found that the files are not always cleaned up automatically. Thus, calling X509Certificate2.Dispose() is a must. This is also mentioned in the MS doc.

The PR updates Kubernetes.Dispose() to dispose certificates.

Closes #1189.

[linux-foundation-easycla]

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• ✅ login: shenglol / name: Shenglong Li (7cd01f1, e869a20)

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[brendandburns]

Why are you deleting this?

[shenglol]

This is dead code. this.ClientCert is never assigned and is always null before the change. I did a git blame and it seems it's a leftover from a previous refactoring. The logic is covered by the code blow.

[brendandburns]

Generally looks good, one comment.

[tg123]

/LGTM

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