Error using graphene source in a local Python neuroglancer

Hi @jbms and @chrisj!

Thanks for adding support for graphen sources - I've been really excited to take this for a spin! Off the bat I'm running into an issue with a locally-run neuroglancer viewer though (probably missing something).

Here's a minimal example:

```Python
>>> import neuroglancer
>>> viewer = neuroglancer.Viewer()
>>> viewer
http://127.0.0.1:57165/v/3f43fc8559322f9408df81a43e42bf881393a163/
```

When I then pull up the browser and add `graphene://middleauth+https://prod.flywire-daf.com/segmentation/1.0/fly_v31` as layer I get this error:

```Python
ERROR:tornado.application:Uncaught exception POST /credentials/3f43fc8559322f9408df81a43e42bf881393a163 (127.0.0.1)
HTTPServerRequest(protocol='http', host='127.0.0.1:57165', method='POST', uri='/credentials/3f43fc8559322f9408df81a43e42bf881393a163', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
File "/Users/philipps/.pyenv/versions/3.9.9/lib/python3.9/site-packages/tornado/web.py", line 1704, in _execute
result = await result
File "/Users/philipps/.pyenv/versions/3.9.9/lib/python3.9/site-packages/neuroglancer/server.py", line 283, in post
provider = self.server._credentials_manager.get(msg['key'], msg.get('parameters'))
File "/Users/philipps/.pyenv/versions/3.9.9/lib/python3.9/site-packages/neuroglancer/credentials_provider.py", line 31, in get
return self._providers[key](parameters)
KeyError: 'middleauthapp'
```

This is using the wheel for `neuroglancer-2.26.post1.dev10` from [build 359](https://github.com/google/neuroglancer/actions/runs/2355513380).

I'm guessing I need to manually add something here? Any ideas?

Thanks,
Philipp