Implement key selectors

[woksin]

Summary

Adds two new event key selectors to projections, StaticKey and KeyFromEventOccurred

Added

• staticKey event key selector attribute for projection On-methods that sets a constant, static, key as the key of the read model
• keyFromEventOccurred event key selector for projection On-methods that uses the event occurred metadata as the key for the projection read models formatted as the string given to the attribute. We currently support these formats:
  • yyyy-MM-dd
  • yyyy-MM
  • yyyy
  • HH:mm:ss
  • hh:mm:ss
  • HH:mm
  • hh:mm
  • HH
  • hh
  • yyyy-MM-dd HH:mm:ss
  • And the above in different orderings