Build a Metaflow flow that fans out with a foreach over a list and joins results back together

Steps

1. Define a FlowSpec with mandatory `start` and `end` steps
2. In `start`, assign a list to an instance variable and call `self.next(self.<step>, foreach='<list_attr>')`
3. In the foreach step, access the per-task item via `self.input`
4. Implement a `join(self, inputs)` step that takes `inputs` and aggregates results (e.g. a list comprehension over `inputs`)
5. Run the flow with `python myflow.py run`