fix(ocpp2): add AvailabilityState defaults and allow self-transitions

fix(ocpp2): add AvailabilityState defaults and allow self-transitions

- Add defaultValue using OCPP20OperationalStatusEnumType.Operative
  enum for ChargingStation AvailabilityState (was missing, causing
  'Missing required configuration key mapping' error at auth time)
- Replace string literals with OCPP20OperationalStatusEnumType enum
  values in AvailabilityState enumeration arrays
- Allow Available→Available self-transitions in OCPP20Constants for
  both ChargingStation and Connector status tables (required by
  boot sequence StatusNotification per spec Part 2 §4.1 step 5)