[codegen][python-experimental] Add configuration knob to disable JSON schema validation

[sebastien-rosset]

1. The 'multipleOf' keyword was not supported for INTEGER values, for any code generator
2. The 'multipleOf' keyword was not supported for python-experimental
3. Add Boolean value indicating whether to disable the JSON
   schema structural validation rules specified in the OpenAPI document.
   This includes multipleOf, maximum, exclusiveMaximum, minimum, exclusiveMinimum,
   maxLength, minLength, pattern...
   By default, the validation is performed for data generated locally by the client
   and data received from the server, independent of any validation performed by
   the server side.
   If the input data does not satisfy the JSON schema validation rules specified
   in the OpenAPI document, an exception is raised.
   If disable_validation is set to true, structural validation is disabled. This
   can be useful to troubleshoot data validation problem, such as when the OpenAPI
   document validation rules do not match the actual API data received by the server.

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[sebastien-rosset]

@spacether , @wing328

[sebastien-rosset]

@wing328

[spacether]

Looks good to me. Thank you for the PR.