feat(wfe-core): add step primitives for workflow control flow

12 step primitives implementing StepBody: DecideStep, IfStep, WhileStep,
ForEachStep, SequenceStep, DelayStep, WaitForStep, ScheduleStep,
RecurStep, PollEndpointStep, SagaContainerStep, EndStep.

Each primitive handles its state machine via persistence_data and
branch creation for container steps.

wfe-core/src/primitives/mod.rs

@@ -0,0 +1,55 @@
+pub mod decide;
+pub mod delay;
+pub mod end_step;
+pub mod foreach_step;
+pub mod if_step;
+pub mod poll_endpoint;
+pub mod recur;
+pub mod saga_container;
+pub mod schedule;
+pub mod sequence;
+pub mod wait_for;
+pub mod while_step;
+
+pub use decide::DecideStep;
+pub use delay::DelayStep;
+pub use end_step::EndStep;
+pub use foreach_step::ForEachStep;
+pub use if_step::IfStep;
+pub use poll_endpoint::PollEndpointStep;
+pub use recur::RecurStep;
+pub use saga_container::SagaContainerStep;
+pub use schedule::ScheduleStep;
+pub use sequence::SequenceStep;
+pub use wait_for::WaitForStep;
+pub use while_step::WhileStep;
+
+#[cfg(test)]
+mod test_helpers {
+    use crate::models::{ExecutionPointer, WorkflowInstance, WorkflowStep};
+    use crate::traits::step::StepExecutionContext;
+    use tokio_util::sync::CancellationToken;
+
+    pub fn make_context<'a>(
+        pointer: &'a ExecutionPointer,
+        step: &'a WorkflowStep,
+        workflow: &'a WorkflowInstance,
+    ) -> StepExecutionContext<'a> {
+        StepExecutionContext {
+            item: None,
+            execution_pointer: pointer,
+            persistence_data: pointer.persistence_data.as_ref(),
+            step,
+            workflow,
+            cancellation_token: CancellationToken::new(),
+        }
+    }
+
+    pub fn default_workflow() -> WorkflowInstance {
+        WorkflowInstance::new("test-workflow", 1, serde_json::json!({}))
+    }
+
+    pub fn default_step() -> WorkflowStep {
+        WorkflowStep::new(0, "TestStep")
+    }
+}