support for async on_transition() and on_dispatch()

[andrew-otiv]

We just tried to make all the methods of our state machine async. For on transition we hit:

  --> bazel-out/k8-fastbuild/bin/otiv3/pipeline_manager_process/state_machines/state_machine.rs:87:21
   |
87 |     on_transition = "Self::on_transition",
   |                     ^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
   |
   = note: expected fn pointer `for<'a, 'b, 'c> fn(&'a mut StateMachineStruct<_>, &'b State, &'c State) -> ()`
                 found fn item `for<'a, 'b, 'c> fn(&'a mut StateMachineStruct<_>, &'b State, &'c State) -> impl Future<Output = ()> {StateMachineStruct::<Handler>::on_transition}`

We get a similar error for on_dispatch. These methods aren't used in your async example. Are they just not supported yet?

Thanks!

[andrew-otiv]

ON_DISPATCH and ON_TRANSITION in the trait IntoStateMachine are non-async.

Thus, the issue isn't just in the macro.

[andrew-otiv]

Draft PR: #29

Draft because at the moment we haven't run the test suite yet.

[mdeloof]

Hi, I've been working on merging the no_std_async branch and there I've made the callback hooks async. on_dispatch and on_transition have been renamed to before_dispatch and after_transition respectively, and two new hooks - after_dispatch and after_transition - have been added.

Feel free to try it out and let me know if you encounter any issues. :)