Add support for OCPP 2.0

[OrangeTux]

@tropxy Already did some good work on adding OCPP 2.0 the enums and structures needed for OCPP 2.0.

But more work has to be done OCPP 2.0 support can be merged to master.

This issue keeps track of the issues related to that goal:

• - Add data classes for the payloads for all call and call results. And implement all enums.
• - Refactor ocpp.v16.charge_point.ChargePoint and abstract out all code that can be shared across both v1.6 and v2.0 chargers. And than implement ocpp.v20.charge_point.ChargePoint based on that shared code.
• - Create OCPP 2.0 examples for both charge point and central system.
• - Update documentation about OCPP 2.0

I suggest to create a feature branch ocpp-20-support. As well created individual tickets for each of points stated above. Each ticket will be worked on in it's on branch. Those branches will me merged into ocpp-20-support. When all tickets are done ocpp-20-support can be merged into master.

[OrangeTux]

OCPP 2.0 has been released as part of version 0.4.0