docs(KEP): add proposal for consolidating the python packages

[zazulam]

Description of your changes: KEP for merging the packages into one.

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[zazulam]

Regarding the tfx question that was brought up in the last community call:

Kfp-pipeline-spec came from support for the tfx use case.

From searching against the tfx repo, it looks like kfp-pipeline-spec is always installed alongside kfp, my thinking is maybe this wouldn't be as big of an issue as we thought.

[mprahl]

@chensun could you please review this?