Implement Debug for ColumnarValue

[iajoiner]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I'm frustrated because ColumnarValue does not have Debug implemented which makes debugging harder.

Describe the solution you'd like
I'd like to implement Debug for ColumnarValue. Since it is already implemented for both Array and ScalarValue it should be very straightforward.

Describe alternatives you've considered
Something like columnar_value.map(|cv| cv.into_array(1)) is doable but it is still not very straightforward.

[alamb]

Proposed fix: #3076