BackendRule_Authentication constructor Null safety

1. spine_client
2. backend.pb
3. BackendRule_Authentication
4. BackendRule_Authentication const constructor

BackendRule_Authentication enum

1. Constructors
2. BackendRule_Authentication
3. Values
4. jwtAudience
5. notSet
6. Properties
7. hashCode
8. index
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==
15. Constants
16. values