ISSUE-136: Support message encryption/decryption

[sijie]

Original Issue: apache#136

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Currently, the nodeJS client does not support message encryption/decryption.

This features is supported in the C++ bindings, so it seems like we only would need to expose the relevant methods as part of the underlying producer config.

[bdkanand]

@sijie : I still see this as Open, did you get it working by any other means?