Add method to convert a data frame to a JSON string

[mtomko]

Adds support for getting data out of a DataFrame as JSON.

The current scala-polars bindings appear to support file I/O, but I couldn't find a good way to load data in to a DataFrame, run some transformations, and then get the results back out for programmatic access. We'd be interested in working on ways to export columns of data as Scala Vector or Array data types as well, but this gets us started.

I welcome suggestions, as I'm a novice when it comes both to Polars and Rust.

[mtomko]

PS, we'd probably also love to have the JSON come back as a byte array and skip the string conversion

[chitralverma]

let me check this out and get back to you by Tuesday

[tmgreen]

Thanks for looking into this, @chitralverma . Are there any updates yet? My team is considering using/contributing to this project but are wondering if anyone is still actively working on it.