Add validation executor

[trumbitta]

Preliminary ideas:

• Create new validation executor to validate the openapi spec (research for best tool for the job)
• Add generator to configure the validation target for api-spec libs
• Add generator to add the validateSpec option to the generate-sources target of api-lib libs (validates the spec before doing anything else, doesn't go through if validation fails)
• Update the api-lib generator to optionally add validateSpec to the generate-sources target
• Update the api-lib executor to validate when validateSpec is true (default: false)

[trumbitta]

Or I think the OpenAPI CLI already can validate specs, so there's that.