Introduce a facility to maintain kubernetes_api. Gitlab::Kubernetes::Client

The only change is to move all Kubernetes-related-kubeclient-methods into separate class Gitlab::Kubernetes::Client to hide a way how we interact with kubeclient and handle errors.

https://gitlab.com/gitlab-org/gitlab-ce/issues/38668#note_47928828

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We introduced a FetchKubernetesTokenService(app/services/ci/fetch_kubernetes_token_service.rb) in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14470. This class is almost copied from KuberntesService(app/models/project_services/kubernetes_service.rb). This is not dry. We should introduce a facility to maintain kubernetes_api and points it from KuberntesService. And we can remove FetchKubernetesTokenService.

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The following discussion from !14470 (merged) should be addressed:

• @dosuken123 started a discussion: (+1 comment)

  @ayufan @nick.thomas This is a class to remove attr_accessor :username, :password from KubernetesService. Previously, we discussed at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14470#note_41830601. I know this is ugly but I don't want to tweak KubernetesService in this MR. I'll create a follow-up MR as ~"technical debt". Then, we can have time to check if the new facility doesn't break any lagacy KubernetesService functionality. What do you think?

/cc @ayufan