Share integration events between projects

From @ReubenBond:

Currently, there are many integration events which have duplicated representations across projects, which makes the complex Order processing flow even more complex because developers cannot use tooling to find usages of the integration event types.

Instead, let's use one or more shared projects to describe these events so that FindAllReferences/etc work.

The current code does make it look like integration events are not cross boundary, as all of the events are being emitted by, and handled in the Ordering API.