validate output of SpecificData.get().getDefaultValue(field) in getSpecificDefaultValue()

scenario:

there's an enum class on the classpath of the same FQCN as an avro enum, but not generated by avro.
avro (at least under 1.9) will return an instance of this non-avro enum out of:

  @Override
  public Object getSpecificDefaultValue(Schema.Field field) {
    return SpecificData.get().getDefaultValue(field);
  }

we could at least validate that the enum instance belongs to a class thats generated by avro, and save users some pain downstream

Start at getSpecificDefaultValue() and inspect its call to SpecificData.get().getDefaultValue(field). Reproduce the classpath case where a same-FQCN non-Avro enum is returned, then define validation that rejects or handles that value; done means the invalid enum cannot pass downstream.