Doc: Add gRPC contract needed to run custom image.

[parshnt]

Summary

Argo Dataflow had a doc that specified the image contract needed to be implemented to run a custom image to process the messages.

https://github.com/argoproj-labs/old-argo-dataflow/blob/main/docs/IMAGE_CONTRACT.md

can we create a similar doc for numaflow?

Use Cases

• While the bundled Go/Java/Python SDKs are good to run custom UDFs. Sometimes we need to implement handler logic in other languages.

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