Support for client-generated IDs

[maurei]

Currently the OpenApi integration takes into account the JsonApiOptions.AllowClientGeneratedIds option when generating schemas for resource objects, but this hasn't been thoroughly tested.

[bkoelman]

AllowClientGeneratedIds was renamed to ClientIdGeneration. It can be set in global options, then overridden per resource type. See https://www.jsonapi.net/usage/options.html#client-generated-ids.