Add Data Validation

[flaxel]

Is your feature request related to a problem? Please describe.
Currently, it is possible to specify null values and non-matching values for a request.

Describe the solution you'd like
The problem can be solved by adding a validation to the rest resource. A possible library is the Hibernate Validator.

Describe alternatives you've considered
A direct implementation with vlingo would be nicer.

Extra:
Further problems with null values could also be solved by writing the code in Kotlin.