Make schema default conversion configurable

[bobbyrauchenberg]

When you give a default for a field in the schema creation, the value needs to be converted to a Java structure

Currently there is a case match on possible types to use for defaults in SchemaHelper.transformDefault - it would be nice to make it possible for users to add their own default conversions - possibly in the shape of a map from DefaultType -> JavaType (which in reality would probably be sth like Map[String, Any] => Any

But this could then be passed in, or added to some config object when schema generation happened