[Core] Add support for task results

[alexrobin]

Sometimes, observations are associated to a particular task/command.

We can extend ICommandStatus to include a result consisting of one or more observations (or observation references).

Results can be made accessible via SPS DescribeResultAccess and SWE API route /commands/{id}/result or /commands/{id}/observations. The SWE API route can report full observations (like a regular datastream), described by a separate schema.

[alexrobin]

One way of implementing this would be the following steps:

• Add result property to ICommandStatus so that a system can associate one or more observations to a status report. This can be the initial status report returned synchronously or any subsequent status reports sent asynchronously. Note that including obs to the initial status report would allow a process to produce its result synchronously.

• Store associated obs either in an existing Datastream or in a dedicated per-task datastream (in a separate obs store = ICommandResultStore?). This would depend if obs was also published to a Datastream event channel or only to the command status channel. Per-task datastreams would not be exposed via API directly.

• When observations are also published to a regular Datastream, only references are kept in the per-task obs store (need specific IObsData implementation).