Allow setting content-type in command

[linxiulei]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Allow setting content-type in command line, which might be preferable for some users to use protobuf than json

Special notes for your reviewer:

Does this PR introduce a user-facing change

For more information on release notes see: https://git.k8s.io/community/contributors/guide/release-notes.md

New command line option `--kube-api-content-type`  is added to specify content type to communicate with apiserver. This option also changes default content type from "application/json" to "application/vnd.kubernetes.protobuf"

[linxiulei]

/assign @x13n

[x13n]

I think it should be possible already to set content type through kubeconfig: https://github.com/kubernetes/client-go/blob/master/rest/config.go#L290 Isn't this sufficient?

[linxiulei]

I think it should be possible already to set content type through kubeconfig: https://github.com/kubernetes/client-go/blob/master/rest/config.go#L290 Isn't this sufficient?

unfortunnately, kubeconfig is parsed with this Config type: https://github.com/kubernetes/client-go/blob/master/tools/clientcmd/api/types.go#L31, which has no content type field

[x13n]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: linxiulei, x13n

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