Support metadata encoding

[Isotr0py]

• Support encode with metadata
• Add encode_with_box to encode image with metadata

By default the encoder assumes no metadata boxes will be added according to libjxl's API. So metadata like EXIF data is missing when creating jxl from png image etc.

An example code to encode with EXIF data:

use std::io::Write;

use jpegxl_rs::encode::{EncoderResult, BoxTypes};
use jpegxl_rs::encoder_builder;
use image::io::Reader as ImageReader;

fn main() {
    let sample = ImageReader::open("sample.png").unwrap().decode().unwrap().to_rgb8();
    let mut encoder = encoder_builder().build().unwrap();

    let file = std::fs::File::open("sample.png").unwrap();
    let mut bufreader = std::io::BufReader::new(&file);
    let exifreader = exif::Reader::new();
    let exif = exifreader.read_from_container(&mut bufreader).unwrap();

    let buffer: EncoderResult<f32> = encoder.encode_with_box(&sample, sample.width(), sample.height(), exif.buf(), BoxTypes::Exif).unwrap();

    let mut file = std::fs::File::create("sample.jxl").unwrap();
    let _ = file.write(&buffer.data);
}