Add Hz as a valid unitOfMeasure

[KoalaWerewolf]

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Is your feature request related to a problem? Please describe. Some devices implement OCPP 1.6 using Hz as the unit for Frequency.

Describe the solution you'd like Add Hz to the json schema in this implementation of OCPP to increase interoperability.

Additional context The OCPP specification advises implementers to make allowance for different implementations when small errors and oversight are discovered where the OCPP specification will not make an update to the schema, e.g. "We advise implementers of an OCPP 1.6 JSON Central System to add the correct spelling (Celsius) to the enumeration. This way your implementation will support both the correct and the incorrect spellings."