Fix bug error panic OCPP 1.6J security features (CentralSystem)

[dwibudut]

Proposed changes

(1) Fix panic when get response proto-error on OCPP 1.6J Security features CentralSystem
(2) Fix connectorId field validation on ExtendedTriggerMessageRequest feature base on OCPP 1.6J security extension specification

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• New feature (non-breaking change which adds functionality)
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Further comments

(1) Example message to produce panic when get response proto-error on OCPP 1.6J Security features CentralSystem

Request message:

[2,"1457709147","ExtendedTriggerMessage",{"requestedMessage":"SignChargePointCertificate"}]

Response message:

[4,"1457709147","NotSupported","Feature not implemented",{}]

CentralSystem will get panic:

...
panic: interface conversion: ocpp.Response is nil, not *extendedtriggermessage.ExtendedTriggerMessageResponse
...

(2) Field connectorId of ExtendedTriggerMessageRequest on OCPP 1.6J Security specification