Client constructor Null safety

1. spine_client
2. client.pb
3. Client
4. Client constructor

Client class

1. Constructors
2. Client
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static properties
12. defaultHost
13. methodSignature
14. oauthScopes
15. Static methods
16. registerAllExtensions