Message registries continuation

[Brad Bishop]

On Tue, 2020-06-16 at 13:39 -0700, James Feist wrote:
> 
> Entity-manager uses valijson, in that way you could validate them 
> against a schema: https://github.com/tristanpenman/valijson. It plays 
> nicely with nlohmann-json. Although if these are compiled in json
> files, I'm not sure it's a large issue. We could just create a
> compile-time script to validate.

FYI for EM, I've actually done this work.  I just need to submit it for
review.