Gateway clusters can have different public URL and private URL

**Describe the issue**:

I am using dask-labextension with Dask Gateway in JupyterHub. The dask gateway dashboard proxy is exposed through an oauth proxy at a public URL. The jupyter-server can talk directly to the proxy service endpoint without auth.

e.g.

- from outside: https://hub.host/services/auth-proxy/dask-gateway/name.uuid/status (needs oauth)
- from jupyter-server: http://traefik-dask-gateway/name.uuid/status (no auth)

which leads to:
- `jupyter-server` _cannot_ talk to the public endpoint because it can't execute oauth.
- browsers _cannot_ talk to the internal endpoint, because it's not a public URL

As a result, the dashboard link in the UI needs to be the public one and the dashboard URL needs to be the proxy one.

I _believe_ it would work if I could tell the labextension dashboard to talk _directly_ to the dashboard URL (no proxy request at all), instead of unconditionally proxying through the extension. I'm not sure of the best way to indicate that, though.

**Anything else we need to know?**:

**Environment**:

- Dask version: 2025.1 (labextension 7.0.0)
- Python version: 3.11
- Operating System: linux
- Install method (conda, pip, source): conda