Resend events, get current context and multi-anchor requires to be resolved

As discussed in the call on 11 oct 2022.

Upon subscription the last -open event is send, the context of this event can be accessed using the GET context method.

In the case of multi-anchor this introduces a race-condition. E.g. when subscribing to Encounter and Patient, it is not clear what open event you will get. As a consequence the complete context is not known.

Proposal:
subscribe - send last -open event(S) for all anchors the Subscriber subscribes to
GET: specialize method url with / and return the context requested for the specfic anchor.

An alternative would be to include all context in the GET call.