Port exec-based client auth provider to DotNet client

[dbenhur]

With 1.10, this exec-based client auth credential provider alpha feature was introduced kubernetes/features#541 for client-go. Exec-based auth plugin should be supported by this DotNet client in addition to client-go. Umbrella issue: kubernetes#62185

/kind feature
/sig api-machinery

[tintoy]

If it helps at all, I implemented this a while back for KubeClient. Happy to contribute my implementation if it'd be useful.

https://github.com/tintoy/dotnet-kube-client/blob/27926d62bf48897837042b624412889fb7b5cdb1/src/KubeClient.Extensions.KubeConfig/K8sConfig.cs#L195
https://github.com/tintoy/dotnet-kube-client/blob/27926d62bf48897837042b624412889fb7b5cdb1/src/KubeClient/KubeApiClient.cs#L146
https://github.com/tintoy/dotnet-kube-client/blob/27926d62bf48897837042b624412889fb7b5cdb1/src/KubeClient/MessageHandlers/CommandBearerTokenHandler.cs

CC: @brendandburns

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[KyleGobel]

Would be nice to have an implementation of this. I'm not sure of a work around to connect to EKS (fairly popular I think?) out-of- cluster without it