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/delay.rs

@@ -0,0 +1,76 @@
+use std::time::Duration;
+
+use async_trait::async_trait;
+
+use crate::models::ExecutionResult;
+use crate::traits::step::{StepBody, StepExecutionContext};
+
+/// A step that sleeps for a specified duration before proceeding.
+pub struct DelayStep {
+    pub duration: Duration,
+}
+
+impl Default for DelayStep {
+    fn default() -> Self {
+        Self {
+            duration: Duration::ZERO,
+        }
+    }
+}
+
+#[async_trait]
+impl StepBody for DelayStep {
+    async fn run(&mut self, context: &StepExecutionContext<'_>) -> crate::Result<ExecutionResult> {
+        // Read duration from step_config if our field is zero.
+        let duration = if self.duration == Duration::ZERO {
+            context
+                .step
+                .step_config
+                .as_ref()
+                .and_then(|c| c.get("duration_millis"))
+                .and_then(|v| v.as_u64())
+                .map(Duration::from_millis)
+                .unwrap_or(self.duration)
+        } else {
+            self.duration
+        };
+        Ok(ExecutionResult::sleep(duration, None))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::ExecutionPointer;
+    use crate::primitives::test_helpers::*;
+
+    #[tokio::test]
+    async fn returns_correct_sleep_duration() {
+        let mut step = DelayStep {
+            duration: Duration::from_secs(60),
+        };
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+        let workflow = default_workflow();
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+
+        let result = step.run(&ctx).await.unwrap();
+        assert!(!result.proceed);
+        assert_eq!(result.sleep_for, Some(Duration::from_secs(60)));
+        assert!(result.persistence_data.is_none());
+    }
+
+    #[tokio::test]
+    async fn returns_zero_duration() {
+        let mut step = DelayStep {
+            duration: Duration::ZERO,
+        };
+        let pointer = ExecutionPointer::new(0);
+        let wf_step = default_step();
+        let workflow = default_workflow();
+        let ctx = make_context(&pointer, &wf_step, &workflow);
+
+        let result = step.run(&ctx).await.unwrap();
+        assert_eq!(result.sleep_for, Some(Duration::ZERO));
+    }
+}