[Java] Create a configurable Formatter for Schemas

The JSON String representation of a Schema is obtained using the toString(...) methods on the class.  Instead of using this method, we should create a configurable Formatter to create the JSON representation.

This would allow configuration options on how the JSON is constructed, notably to include/exclude schemas that should be considered "known" when the Schema is parsed (see AVRO-2822).

This is similar to how the old parse(...) methods directly on Schema were deprecated in favour of a Parser.