Function to authenticate flow requests

[byrichardpowell]

WHY are these changes introduced?

We currently don't support Flow extension requests properly. While they use an HMAC signature similar to webhooks, they don't have all of the same headers, so their validation is slightly different.

WHAT is this pull request doing?

Adding a new way of supporting Flow extension requests under shopify.flow.validate.

Type of change

• Minor: New feature (non-breaking change which adds functionality)

Checklist

• I have used yarn changeset to create a draft changelog entry (do NOT update the CHANGELOG.md file manually)
• I have added/updated tests for this change
• I have documented new APIs/updated the documentation for modified APIs (for public APIs)

[paulomarg]

MD link checks are failing because the link from the changelog to the docs will only be correct after this is merged.

Ended up updating the process to ignore changeset files during the link checks. If any broken links are introduced, they'll show up in the release PRs :)