example: turn directory into UnixFS DAG

## Problem

We have many unixfs implementations, and it is extremely confusing for new developers to understand what they should use and why. (cc @2color)

## Proposed improvement

The [./exmaples](https://github.com/ipfs/boxo/tree/main/examples) should have an example of `ipfs add + ipfs dag export` equivalent for reading a file or a directory from local filesystem, and turning it into a CAR with UnixFS DAG that was properly chunked / HAMT-sharded.

The example should be a CLI tool that accepts same flags as `ipfs add`, demonstrating how to adjust chunker, hash function, or decide if mode/mtime should be preserved.