Add option to disable validation against JSON schemas

[OrangeTux]

TMH has integrated a charge point which isn't fully compliant with the OCPP 1.6 specification. The MeterValues requests send by the charge point contains an 'illegal' measurand. Receiving a MeterValues request will fail with:

Traceback (most recent call last):
File "/venv/lib/python3.7/site-packages/ocpp/messages.py", line 127, in validate_payload
     validate(message.payload, schema)
    File "/venv/lib/python3.7/site-packages/jsonschema/validators.py", line 899, in validate
      raise error
  jsonschema.exceptions.ValidationError: 'Request.Energy' is not one of ['Energy.Active.Export.Register', 'Energy.Active.Import.Register', 'Energy.Reactive.Export.Register', 'Energy.Reactive.Import.Register', 'Energy.Active.Export.Interval', 'Energy.Active.Import.Interval', 'Energy.Reactive.Export.Interval', 'Energy.Reactive.Import.Interval', 'Power.Active.Export', 'Power.Active.Import', 'Power.Offered', 'Power.Reactive.Export', 'Power.Reactive.Import', 'Power.Factor', 'Current.Import', 'Current.Export', 'Current.Offered', 'Voltage', 'Frequency', 'Temperature', 'SoC', 'RPM']

In order to support chargers like this it will be possible to disable JSON schema validation of incoming requests. The validation can be disabled by passing skip_schema_validation=True to a @on decorator. This will disable validation for that specific request and the response.

@on(MeterValues, skip_schema_validation=True)
def on_meter_values(self, *args, **kwargs):
    pass