[Feature] Allow for relative urls in Flow action extensions

[EliasJRH]

WHY are these changes introduced?

Part of https://github.com/Shopify/flow/issues/21604. To start allowing the usage of URLs relative to an apps application_url in Flow action extensions.

WHAT is this pull request doing?

This pull request extends the capabilities of Flow action extensions to allow them to start using relative runtime_urls in their config.

How to test your changes?

• Create a Flow action extension with a relative runtime_url (begins with a /)
• Create a Flow workflow using action extension
• Run shopify app dev to spin up cloudflare tunnel
• Execute workflow
• Observe that Flow action extension hits cloudflare tunnel at specified relative URL

Post-release steps

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add