[FEATURE] Expose the tpchgen-cli as a rust library

[guillesd]

Is your feature request related to a problem? Please describe.
If I want to use the tpchgen-cli functionality (since it is the simplest entrypoint), I need to cook my own thing with tpchgen and tpchgen-arrow. It would be very nice if tpchgen-cli is also exposed as an API to rust!

Describe the solution you'd like
Expose this function https://github.com/clflushopt/tpchgen-rs/blob/main/tpchgen-cli/src/main.rs#L250 and the CLI config struct so that I can do something like:

let tpch_runner = Cli {
    scale_factor: 1.0,
    output_dir: PathBuf::from("./data"),
    tables: Some(vec![Table::Customer, Table::Orders]), // assuming Table enum variants
    parts: Some(1),
    part: None,
    format: OutputFormat::Tbl,
    num_threads: 4,
    parquet_compression: Compression::SNAPPY,
    verbose: true,
    stdout: false,
    parquet_row_group_bytes: 10_000_000, // 10 MB
};
tpch_runner.run()?; // or similar

Describe alternatives you've considered
Copy all the CLI implementation into my own application, which seems waistful.

[clflushopt]

Hi @guillesd thanks for opening this issue and yes that's a good suggestion ! We've actually discussed this a while back as well IIRC but we didn't flesh out a good API for it. I think it's the right time to do so.

[guillesd]

I mean I think the CLI API is pretty good, it does exactly what I need (just the naming is a bit weird maybe). But cool to see that you agree this would be a nice to have! Let me know if I can help :)

[clflushopt]

@guillesd what changes or suggestions do you have for naming and which part do you find weird, curious to know !

[guillesd]

What I mean is that maybe it is weird that, if you use the crate as a library, you use the struct CLI, but you are not using it as a CLI. I don't think there is anything weird right now, just saying that if you use this form your Rust project like so:

let tpch_runner = Cli {...};
tpch_runner.main()?;

Maybe it looks better to do

let tpch_runner = Runner {...};
tpch_runner.run()?;

But I also don't have any problems with the former!

[clflushopt]

A tentative PR is available @guillesd reviews/comments welcome if you have the time #202.

cc @alamb I think this would benefit from your input as it was something we discussed in the past