improve extension

[matt-beanland]

We don't have spark cheat sheets for the Diffo.Provider.Instance.Extension

Also the Assigner should be moved from Diffo.Access to Diffo.Provider
Would be nice to allow unregistered resources too, so livebook can create resources/domains at will.

[matt-beanland]

warning: Entity without spark_metadata field is deprecated. Entity Diffo.Provider.Instance.Feature does not define a __spark_metadata__ field
warning: Entity without spark_metadata field is deprecated. Entity Diffo.Provider.Instance.Characteristic does not define a __spark_metadata__ field