[Draft] Improve projection user experience

[ethe]

use magic number to specify projection columns makes user confuse:

...
let name = "Alice".into();
let upper = "Bob".into();
let mut scan = txn
    .scan((Bound::Included(&name), Bound::Excluded(&upper)))
    // what `1` means???
    .projection(vec![1])
    .take()
    .await
    .unwrap();
...

[dharanad]

To get started with the project, I would like to pick this issue up. I know its in Draft, but please do share more info sooner later

[ethe]

@dharanad sure, I will complete it, also if you have good ideas about this design, please generously share it 😄