Refactor DataTransferRequestedEvent

Refactored DataTransferRequestedEvent to support the DataTransferMessage
type.
Refactored OcppJsonService

Refactored OcppJsonService to use the new DataTransferMessage type.
Refactor ChargingStationOcpp15Client

Refactored ChargingStationOcpp15Client to use the new
DataTransferMessage type.
Fix JavaDoc comments

Fixed JavaDoc comments for data transfer related commands and events.
Fix spelling of "successful"

"Successful" was spelled "successfull" in a couple of places. This
commit fixes that spelling error.
Delete duplicated Day enum from VAS

The VAS component had a copied Day enum to store the day of an opening
time. This commit deletes that copied code.