Support configuration of custom GenericData/SpecificData instances

[ivangreene]

Some Avro users may wish to extend GenericData/SpecificData, for example to configure a set of logical type conversions or to add handling for custom types.

Many Avro methods and class constructors used during ser/des optionally allow passing a specific instance of GenericData or SpecificData (e.g, the constructors of {Generic,Specific}Datum{Reader,Writer}). We should take advantage of this and allow the user of schema registry to configure a supplier of Generic/SpecificData instances, which would default to Generic/SpecificData.get()