Unreference the schema.

Unreference the schema, replacing the OpenApi.IJsonSchema.IReference type to the actual schema. Different with escape is, the unreference function does not resolve every references in the schema, but resolve only one time.

If there's a OpenApi.IJsonSchema.IReference type which cannot find the matched type in the OpenApi.IComponents.schemas, and you've called this unreference() function with the reference, it would also be failed and return an IOpenApiSchemaError value.