Add already parsed body JsonBody to CustomRequestValidator

[Sven Döring]

The CustomRequestValidator enables custom, additional request validation to the request headers, parameters and body.

For the body validation it may be necessary to parse the body to custom-validate its values. That is unnecessarily unperformant as the body already has been parsed in the RequestBodyValidator.

May it be possible to cache the JsonNode result for the custom validation? Maybe directly in the Body#toJsonNode() method?