Implemented yaml parsing for config file.

[dragosnutu]

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Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

It implements #2425 support for yaml. It basically checks if the input file has yaml or yml extension, if so it uses the Yaml.mapper() for this. If any other extension, or empty (so no breaking change) it will use the old Json.mapper().

[wing328]

@dragosnutu thanks for the enhancement, which looks good to me

cc @OpenAPITools/generator-core-team

[dragosnutu]

@wing328 I'll try to add also the other idea to for the config-help to generate sample configurations with default value, maybe on another PR.