[workflows] adding restart-from-step support to the sdk

[vaishnav-mk]

Workflows: Add restart from step support for local dev

Extends WorkflowHandle.restart() to accept an optional from parameter for selective restart

Describe your change...

Adds support for:
await instance.restart({ from: { name: 'aggregate' } });
await instance.restart({ from: { name: 'process', count: 3 } });

---

• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
    • Ran fixtures/workflow and fixtures/workflow-multiple locally with wrangler dev, triggered restarts with from param via the binding, verified preserved steps retain cached outputs and re-executed steps produce fresh results - [ ] Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: API surface matches workerd types, docs will be updated with the workerd PR

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