Add support for tuple items

[hamzaed]

The current implementation of the metamodel uses items to set the type of the items of an array in a Schema definition.

However, even though it's not common in OpenAPI definitions, JSON schema allows the definition of tuples by setting the value of items to an array where each item is a Schema which corresponds to the type of each index.

The importation fails when `items``contains an array (tuple definition) instead of an object (array definition).