Refactor Ocpp 1.6/2.0

[lorenzodonini]

Refactored existing OCPP 1.6 / 2.0 structure and naming

Breaking changes:

• Changed Confirmation interface name to Response
• Changed Confirmation implementation names in OCPP 2.0 package to Response
• Moved OCPP 1.6 and 2.0 profiles (functional blocks) into separate packages
• Replaced core profile/handler for OCPP 2.0 with functional profiles/handlers
• Moved shared OCPP 1.6 types into a separate types package
• Moved shared OCPP 2.0 types into a separate types package
• Changed all listener names to handler
• Rename CentralSystem and ChargePoint OCPP-J structs to Server and Client respectively

Additions & Improvements:

• Moved some logic from v16.go and v2.go into separate files
• Use RFC3339 standard for dateTime formatting in OCPP 1.6
• Add MessageTypeNotSupported error code in OCPP-J
• Check for unsupported feature when sending a request
• Cleanup code
• Improve documentation for OCPP-J, OCPP 1.6 and OCPP 2.0