NotebookNotary does not accept data_dir config option

The [docs](https://jupyter-notebook.readthedocs.io/en/stable/security.html?highlight=data_dir) specify I should be able to set the notary's data_dir as:

```
c.NotebookNotary.data_dir = "/path/to/signature_dir"
```

but this produces `[TrustNotebookApp] WARNING | Config option 'data_dir' not recognized by 'NotebookNotary'`

`jupyter notebook --generate-config` and `--help-all` also do not specify `data_dir` as an allowable config option. This seems to be caused by https://github.com/jupyter/nbformat/blob/a06f4c84738b338fee5ad6316b21918a8709b636/nbformat/sign.py#L315
not having something like:
```python
data_dir = Unicode(
help="""The storage directory for notary secret and database."""
).tag(config=True)
```

For reference, I am trying to share a set of trusted notebooks amongst many users, and was hoping I could point all users at some shared directory location.

Start in nbformat/sign.py near the NotebookNotary configuration referenced in the issue and compare its options with the documented data_dir setting. Verify the configuration through jupyter notebook --generate-config and --help-all, then confirm that setting a shared directory is accepted without the warning.

Written by the indexing model from the issue text.