`meta` and `errors` should be annotable

[hoehrmann]

In https://github.com/sdmx-twg/sdmx-json/tree/71fe5eaa9fcd29e3c15f2f0216a19b9b650b1dbd it seems fairly inconsistent that almost anything can have annotations, but meta and errors cannot. Implementers who would like to store additional data there would have to use extension properties, which then have interoperability problems since different implementations are likely to use incompatible conventions. In contrast, implementations are far more likely to converge on common terms using annotations (for instance, implementations might want to indicate the software and version that produced a message for informational and debugging purposes, and new implementations would likely use the same annotation that existing implementations use). So I think annotations should be supported in these locations.

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

[dosse]

@hoehrmann Thanks for suggestion! If accepted then this would be something also to be done for SDMX-ML. Note that annotations are part of the SDMX information model while message headers/footers/meta/errors are not part of it.