V2 backcompatibility for V3

[jpwilliams]

Summary

Adds backwards compatibility for in-progress v2.x.x runs for v3.x.x SDKs.

• Adding x-inngest-req-version header appropriately from response
• v0.test.ts
• Lazily importing backwards compatibility N/A Tiny foot-print; can optionally add later

Checklist

• Added a docs PR that references this PR N/A
• Added unit/integration tests
• Added changesets if applicable N/A

Related

• V3 #294

[changeset-bot]

⚠️ No Changeset found

Latest commit: f3b9396

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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