Add SOP Class UID to ChangeFeedEntry

User story
As a consumer of the change feed, I would like to process or discard a received instance according to its SOP Class.

Currently, this can be done by retrieving all the meta-data and extracting the SOP Class from it. However, adding SOP Class to the ChargeFeedEntry itself would mean that the feed could then be used without the meta-data fetch, leading to improved performance.

Although extension of ChangeFeedEntry should not be done lightly, I think processing decisions based upon SOP Class UID would be a common usage scenario.

In our scenario, for example, we want to ignore all CT Image instances (or any other image), and process only RT Structure Set, RT Plan, and RT Dose instances (so just 2 or 3 instances per 200-or-so instance study in this case).

Acceptance criteria

1. SopClassUid is available as a string attribute in each returned ChangeFeedEntry.

Start at the ChangeFeedEntry model and follow the change feed response entry point to see where returned attributes are assembled. Add the SopClassUid string attribute and verify that every returned ChangeFeedEntry exposes it, including the acceptance scenario of filtering instances by SOP Class UID.