Allow ArrowCSV writer to control the display of NULL values

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I am working on sqllogictest based testing in DataFusion - apache/datafusion#4460

We are using the Arrow CSV writer 👍

However, in order to implement the spec correctly, we need to display NULLs as "NULL"

Currently the CSV writer writes out NULLs as empty strings ""

I would like a way to control what string was used.

Describe the solution you'd like
Extend https://docs.rs/arrow-csv/28.0.0/arrow_csv/writer/struct.WriterBuilder.html

        let builder = WriterBuilder::new()
          .has_headers(false)
          .with_delimiter(b' ')
          // new setting to control NULL output
          .with_null(b'NULL');

So instead of wrting out the values `A, null, B' as

A

B

As is done today this would be written like

A
NULL
B

Describe alternatives you've considered
I could effectively fork the entire CSV writer into the datafusion testing harness and make it do what I want but I would rather get it in arrow.

Additional context
apache/datafusion#4500

[askoa]

I'll pick this up.