example: turn directory into a CAR with UnixFS DAG

[lidel]

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 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.

[lidel]

@gammazero thoughts which APIs should be used in this example?