TransactionOptions_Mode constructor Null safety

1. spine_client
2. common.pb
3. TransactionOptions_Mode
4. TransactionOptions_Mode const constructor

TransactionOptions_Mode enum

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