CalendarEvent/maybe - Create data source & repo

[tddang-linagora]

Definitions of done

• Create abstract data source & repo, taking in list of blob id and returning response model created from Create calendar event maybe method jmap-dart-client#82
• Create Calendar event API directly linked to calendar event maybe method from Create calendar event maybe method jmap-dart-client#82
• Implement calendar event data source, utilizing the above API
• Implement calendar event repo, utilizing the above data source
• Write unit tests for repo implementation

Notes
The data source doesn't need unit tests here because the tests are already written in linagora/jmap-dart-client#82