Retrieve Event based on Url (or other attributes)

[nickrandolph]

(see #132 for background on this request from @aloisdeniel)

A great addition would be to be able to retrieve an event from its url too! :)

Because with current APIs we need to load all events to check if it already exists.

final events = await systemCalendar.retrieveEvents(calendarId, RetrieveEventsParams(
eventUrls : ["custom://events/563"],
));