Refactor to reduce duplication in the parseEvent method

At the moment, significant code within the `parseEvent` method (see e.g [`normalise-event-pipe.js`](https://github.com/openactive/harvester/blob/virtual-events/src/lib/pipes/normalise-event-pipe.js#L58) is duplicated across all top-level Pipes. This should be refactored into a helper function or shared library.