Standardize urn:problem-type:cbss:input-validation:* issue types under Belgif

[jpraet]

The InputValidationIssues class lists a bunch of general issue types that are currently standardized within CBSS (urn:problem-type:cbss:input-validation:*).

These are not CBSS-specific, but rather handle general validation issues, validation issues of Belgif-standardized openapi types, and cross-parameter validation issues.

So these are all candidates to be standardized under urn:problem-type:belgif:input-validation:*.

See also:

• Standardize validation issues on belgif types rest-guide#126
• Standardizing some common issue types rest-guide#113