Make RetryExhaustedOnAction and TimeoutOnAction strings accessible from Tree

[TravisOnGit]

Instead of hard-coding "RetryExhaustedOnAction" and "TimeoutOnAction", let's add public const strings or enums so they are more accessible. They should be accessible through the tree/schema so users can easily check the ActionResponse.Status.

Possible solution is to add a new class to the parameters of the Roslyn script.

"ShouldSelect": "C#|(await Session.GetLastActionResponseAsync()).Status == FooEnums.TimeoutOnAction",