Support opening a pipeline via frame callback

[w-utter]

This would add the ability to open a pipeline by providing a callback by using the rs2_pipeline_start_with_callback sys fn. Its usage would look something along the lines of

use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;

let context = Context::new().unwrap();
let pipeline = InactivePipeline::try_from(&context).unwrap();

let frame_count = Arc::new(AtomicUsize::new(0));

let stream_handle = pipeline.start_streaming(move |frame| {
    let frame_count = frame_count.clone();
    frame_count.fetch_add(1, Ordering::Relaxed);
    let _frame = frame.of_type::<CompositeFrame>().unwrap();
})