refactor(iroh-bytes): Update bao-tree to 0.12 and adjust code (#2153)

## Description

refactor(iroh-bytes): Update bao-tree to 0.12 and adjust code

bao-tree has some API changes that need to be adapted. Outboard creation
is easier, and the ByteNum newtype is gone, replaced by just u64.

## Notes & open questions

~~There is still a test failure. Hopefully nothing major. But hence the
draft status.~~

## Change checklist

- [ ] Self-review.
- [ ] Documentation updates if relevant.
- [ ] Tests if relevant.

Author: rklaehn