Make possible to pass a cancellation token

[francotiveron]

I propose to add an option to the configuration to generate code with CancellationToken for asynchronous calls.
For example, asyncReturnType could have 4 possible values instead of 2:
["asyncReturnType"]: <"async" | "task" | "async-ct" | "task-ct">

[francotiveron]

See #47
Please note that, while this work for me, It probably needs more extensive testing.

If accepted, the readme needs also to be updated. (Happy to do that).