[core] consider content in parameters and headers when computing unused schemas

[jmini]

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Description of the PR

Introduced with #74 and updated with #1232 and #2138, the ModelUtils.getUnusedSchemas(OpenAPI) returns all schemas that are not used in an OpenAPI specification.

This PR fixes the case where a Schema is referenced in an Content of and Header or Parameter.

Original issue: #50

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[wing328]

LGTM