No `version.json` to match go-ipld-prime

go-ipld-prime has a `version.json` file. py-ipld-dag should have one for consistency across the multiformats ecosystem.

### Proposed Solution

1. Create `version.json`:
```json
{
"version": "v0.2.1"
}
```

2. Add to `bump-my-version` config in `pyproject.toml`.

### Related

- Go equivalent: go-ipld-prime `version.json`

Start by reading pyproject.toml, especially the bump-my-version configuration, and compare the repository's versioning conventions with the linked go-ipld-prime version.json. Create version.json with the proposed version and update the bump-my-version configuration; done means both files are consistent and version bumps can include the new file.

Written by the indexing model from the issue text.