Interface that defines API for simple extensions that can provide additional deserializers for deserializer Map keys of various types, from JSON property names. Access is by a single callback method; instance is to either return a configured KeyDeserializer for specified type, or null to indicate that it does not support handling of the type. In latter case, further calls can be made for other providers; in former case returned key deserializer is used for handling of key instances of specified type.