SDMX Registry types (or entire specification for registry messages) missing.

[hoehrmann]

The SDMX-REST specification refers to types like SubmitStructureResponseType. Considering the default format in most other places of SDMX-REST is SDMX-JSON, it seems weird that one would have to use SDMX-ML there, when a web service responds to a submission in SDMX-JSON. Those types however do not seem to be mentioned in this repository.

I take it this is already mentioned in #63.

(In doubt, please handle this as a public review comment on SDMX 3.1 once the comment period begins.)