[PROPOSAL] External Join with KV Stores

[JingsongLee]

Hi all:
In some scenarios, the user needs to query some information from external kv 
store in the pipeline.I think we can have a good abstraction that allows users 
to get as little as possible with the underlying details.Here is a docs of this 
proposal, would like to receive your feedback.
https://docs.google.com/document/d/1B-XnUwXh64lbswRieckU0BxtygSV58hysqZbpZmk03A/edit?usp=sharing
Best, Jingsong Lee