add xgboost job python sdk

[data-infra]

this is xgboost job python sdk, developers can import package to deploy xgboost on kubernetes(like tfjob)

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[kubeflow-bot]

This change is 

[k8s-ci-robot]

Hi @626626cdllp. Thanks for your PR.

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[terrytangyuan]

Looks like you are directly modifying from Python SDKs for other operators created by @jinchihe? Have you verified that this works?

[terrytangyuan]

Where are these markdown files?