Implement ArrowSerialize/ArrowDeserialize for Utf8Scalar

[emilk]

Utf8Scalar is similar to Option<String> except they are refcounted, and thus support O(1) cloning and slicing.

In other words, this should work:

use arrow2_convert::{ArrowDeserialize, ArrowField, ArrowSerialize};

#[derive(ArrowField, ArrowSerialize, ArrowDeserialize)]
pub struct Foo {
    pub string: arrow2::scalar::Utf8Scalar<i32>,
}