Full UnixFS DAG construction support

[b5]

Tracking issue to follow our support for creating UnixFS files, directories with iroh

• Create files of "any" size
• Create files within a directory
• Proper file size accounting Write blocksizes & filesize for unixfs nodes #202 feat(unixfs): add filesize and blocksizes for UnixfsNode::File nodes iroh#254
• Recursive subdirectories kubo add -r $DIR
• Can create symlinks
• Exposed as a library-level primitive
• Integration test that confirms kubo can open a multi-block file in doubly-nested directory created with iroh
• adding works via irohctl