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.