netstandard2 extendable lib model, basic rest

[tg123]

The problem

• old runtime support
  the kubernetes .net sdk is dropping support of old .net runtimes because of some soft-required features missing. for exmaple, websocket apis are missing in netstandard2 and certificates loading api are only in net5 or later. Framework no longer supported #743

  however, it is difficult to upgrade .net runtime for many tricky reasons. it is time to welcome those users in future sdk versions.

• generated model library
  feature request from many internal ms team or external users. mostly because a slim client is enough for their case and the fancy features introduces many unwanted dependencies.

The proposal

the idea is to break the sdk into multiple smaller library:

• KubernetesClient.Models (netstandard2): generated models
• KubernetesClient.Models.Versioning (netstandard2): generated models version converter with injectable
• KubernetesClient.Basic (netstandard2): generated rest stub, this is an abstract class, http call related must be implemented in runtime specific libraries.
• KubernetesClient (net6+): full featured package for most users
• KubernetesClient.Net48 (net48): to support LTS windows server users until net48 eol
• KubernetesClient.Auth.XXX (netstandard2, net6?): cloud specific token generator
• KubernetesClient.Kubectl (net6): kubectl like api
• ....

bold items are current in this PR, others are planned only
users can build highly customized library with basic generated model and vanillarest libraries.

[tg123]

#743
#448

[brendandburns]

/lgtm
/approve

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