Improve the ergonomics of creating field and list array accesses

[alamb]

Is your feature request related to a problem or challenge?

#6936 from @izveigor added a really nice set of array functionality 🏆 to access individual array elements as well as ranges!

However when I was testing the a bit of a mismatch in the GetFieldAccess struct which is now overloaded to return a named field in a struct, an element in a list or a range of a list.

More commentary here #6936 (comment)

You can see for example, the code to manipulate it here is pretty tricky: https://github.com/influxdata/influxdb_iox/pull/8419/files#diff-2ea572c49a95b9cbfa0647dbb38795bff1aa4dc788518bc8e83f80814f061b1f

Describe the solution you'd like

I think it would be a clearer API to separate out the FieldAccess for struct fields and ListAccess for list element acess

Perhaps something like

enum GetFieldAccess {
  /// returns the field `struct[field]`. For example `struct["name"]`
  NamedStructField { 
    name: ScalarValue,
  },
  /// single list index
  // list[i]
  ListIndex: {
    key: Expr
  },
  /// list range `list[i:j]`
  ListRange: {
    start: Expr
    stop: Expr
  },
}

Describe alternatives you've considered

No response

Additional context

No response

[izveigor]

@alamb The implementation via enum is actually a great idea! Can I take this ticket if no one minds?
And also I think we can improve our spaghetti-code in array_expressions. It maybe we can design some patterns to describe all actions with arrays 🤔. What do you think about it, @alamb?

[alamb]

@izveigor that would be great if you are willing 🙏

I am somewhat interested in getting this in before we release DataFusion 29 (probably at least a week away) so we don't release the current structures and thus have two breaking changes

Thus if you have time to work on it soon this would be great. Otherwise I will be happy to do this work as well