[#26] Perform a comprehensive review of KZOP validations vs those called for in the specification

[andylowry]

Steps:

1. Catalog all validations in a YAML document
2. Generate reports and whatever code may be helpful from the catalog (e.g. creating a means of ensuring that every level-3 validation is represented in the code).
3. Remove explicit "required" property checks by incorporating requiredness into the types model and automating them (except where requiredness is contextual, e.g. in SecurityScheme).
4. Correct existing validations as needed, tag them with validation ids, and add missing validations.
5. (Hopefully in coordination with the OAS compliance initiative) Develop tests of all level-3 validations, and representative level-2 validations (assuming they are all automated), checking both for false negatives and false positives.

Not all the above will necessarily be done as part of this PR.