Context for forwarding requests to be stored in the registration in mongo

[kzangeli]

No context is stored as part of a registration right now - it's missing functionality.
In the future, a special payload body field (yet to be named in ETSI ISG CIM) of a registration is to assign an @context to be used when forwarding triggered by the registration.
Right now (and, Orion-LD hasn't implemented that EITHER! - only for subs/notifs) the @context used during the registration creation is supposed to be stored inside the registration in the DB. But, if we do that, then you can't create the reg with one @context have another @context to be used when forwarded messages are triggered.

So, a new field, perhaps called "ldContext" is to be added to the Registration and this field is to be stoired in the DB and it is to be PATCHable.