Support more complex input relation checks in RequestValidator

[clone1612]

Consider an API with query parameters sector & institution that have the following relation:

• Both can be absent
• If institution is provided sector should be as well
• Sector can be provided without an institution

The current methods from RequestValidator cannot express this, the closest we have is when(boolean condition, Consumer<RequestValidator> requestValidatorConsumer) but that doesn't result in a response which expresses the relation.

I believe adding a method for this scenario would be useful, although I don't have an immediate idea for its name. Maybe requireDependency ?

[jpraet]

Yes, I think this type of check may be worth supporting / standardizing. This study states it as the most common type of cross-parameter validation.

But the standardization process of these cross-parameter input validation issues has not been going smoothly as it is. I'm therefore a bit hesitant on adding yet another issue type to the proposal.

Perhaps we can implement it without introducing a new issue type, and reuse the existing urn:problem-type:belgif:input-validation:requiredInput = Input is required in this context. With a detail message that expresses the relation between both parameters, and referencing both parameters in the (also not standardized) inputs[] array.

@pvdbosch what do you think?