[Go] add method on Array interface

I think we need method `Values() []interface{}` on interface [array](https://github.com/apache/arrow/blob/7e7b8e1f0a2f02046e0d0d6a0b9ada6d64648b9c/go/arrow/array.go#L87) to return values from data/array

Start in go/arrow/array.go at the Array interface, then trace the implementations under data/array. Determine which implementations need to expose Values() []interface{} and how their existing value-access methods represent data. Done means the interface and implementations provide the method consistently and the Go package still builds and passes its tests.

Written by the indexing model from the issue text.