OCPP 2.1 support

[robert-s-ubi]

Time to get rolling with OCPP 2.1!

PR #405 adds the OCPP 2.1 messages and types, and adds OCPP 2.1 to the protocol version enum, which is almost everything needed to fully implement OCPP 2.1.

What's missing:

1. NotifyEventStream support. This is a new type of unidirectional message introduced in OCPP 2.1, which uses a new RPC MessageType "SEND". The problem with this is that the library currently has no mechanism to send a message without expecting a response, nor to handle an incoming message without sending anything back, so API changes/extensions are probably needed for this.

2. OCPP 2.1 introduces another new RPC MessageType "CALLRESULTERROR" to report back errors which occurred while processing a "CALLRESULT". The library currently sends a "CALLERROR" in this situation, which is actually not foreseen in any OCPP version, and will fail OCPP certification tests. So the code should be fixed to send "CALLRESULTERROR" if connected with OCPP 2.1, and nothing if connected with any earlier OCPP version.

[robert-s-ubi]

OCPP 2.1 support is complete!

I have just added some commits to PR #405, which complete the OCPP 2.1 support and add some tests to verify the new capabilities work as expected, as well as showcasing how they are used.

With this, the library supports all features of OCPP 2.1, including NotifyPeriodicEventStream and CALLRESULTERROR.

Happy Christmas Everyone!