docs: Add a KEP for a Kubernetes native API for pipelines

[mprahl]

Description of your changes:

This proposesd a Kubernetes native API for pipelines and pipeline versions. This uses the KEP format in anticipation of #11535.

Here is a demo of a prototype with this design.

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[mprahl]

/cc @chensun @zijianjoy

Could you please review this when you get a chance?