Implement string_concat kernel

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
We currently have a concat kernel at https://docs.rs/arrow/11.1.0/arrow/compute/kernels/concat/index.html

This concatenates Arrays together.

There is also a need to concatenate strings together. Something like string_concat that takes two StringArrays (or LargeStringArrays` and concatenates them element by element.

DataFusion has an implementation of string concat here: https://github.com/apache/arrow-datafusion/blob/28a6da3d2d175eb9d2f4ff8a6ea58e7c22dae97c/datafusion/physical-expr/src/expressions/binary.rs#L422 which @WinkerDu has kindly been improving.

Describe the solution you'd like
I suggest adding an optimized string_concat kernel in arrow-rs. @Dandandan outlines some good first optimizations here: apache/datafusion#2183 (comment)

The signature would be something like:

pub fn string_concat<OffsetSize: StringOffsetSizeTrait>(
    left: &GenericStringArray<OffsetSize>, 
    right: &GenericStringArray<OffsetSize>
) -> Result<GenericStringArray<OffsetSize>>

An example use:

    let left = [Some("foo"), Some("bar"), None].into_iter().collect::<StringArray>();
    let right = [None, Some("yyy"), Some("zzz")].into_iter().collect::<StringArray>();

    let res = string_concat(left, right).unwrap();

    let expected = [None, Some("baryyy"), None].into_iter().collect::<StringArray>();
    assert_eq(res, expected);

Describe alternatives you've considered

Additional context
Add any other context or screenshots about the feature request here.