Mark methods as override

[kromahn]

Describe your changes

To improve code readability and maintenance, all overriding methods are marked with keyword override (allows to enable warning-free the compile options "-Wsuggest-override").

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• If OCPP 2.0.1: I have updated the OCPP 2.0.1 status document
• I read the contribution documentation and made sure that my changes meet its requirements

[marcemmers]

Hi @kromahn,

Thanks for the contribution. Looks like a good improvement!
We do have some code generators in place that can generate these message definitions, could you update those as well?

They're located in src/code_generator/common/templates/. I expect that once you update that the output will look the same.