[feature] Added endpoint to list all Radius Groups #553

[pranshustuff]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [x] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #553.

Description of Changes

Added an endpoint /radius/group to list all Radius Groups. Also added a test to check the working of the endpoint.

Screenshot