Add OCPP 2.1 support

[robert-s-ubi]

Add Java classes for all OCPP 2.1 messages and types as well as client and server event handler interfaces and function classes corresponding to the OCPP 2.1 "functional blocks."

Add OCPP 2.1 to the ProtocolVersion enum.

Specify UTF-8 encoding for the Java source files in build.gradle to avoid compiler warnings about the UTF-8 sequences in the source files.