[feature] API Endpoint to List Radius Groups

### Discussed in https://github.com/openwisp/openwisp-radius/discussions/552

<div type='discussions-op-text'>

<sup>Originally posted by **3pleo** September 18, 2024</sup>
Hi,

I can't seem to find any API endpoint to list the configured Radius Groups (/admin/openwisp_radius/radiusgroup/) in openwisp api/v1/docs/

How can I achieve this?

Thanks.
Regards.</div>