Add support for control over which validations are applied

[James Navin]

To support cases where there a known deficiencies in a spec (e.g. it is incomplete, under development, or does not include new resources) it would be good to be able to choose which validations are applied so that clients do not become blocked in these situations but can still get benefit from validation.

Examples:

• Skip required field validation for responses
• Skip parameter format validation
• Skip unknown path validation
• Skip unknown response code validation