Add mrt-utilities

[kieran-sf]

Summary

Adds the new mrt-utilities package containing:

• Middleware for simulating a deployed MRT environment in local development
• The MRT streaming handler

Testing

• Checkout this managed-runtime-libraries PR
• Run the script to import these mrt-utilities pnpm link-mrt-utilities
• Manually run the reference app locally and test the middleware
  • Make a request with ?hello=world&removeme=whatever
  • Make a request to /mobify/bundle/1/saturn.png = 404
  • Make a request to /mobify/bundle/1/saturn.jpg = picture of saturn
  • Make a request to /mobify/proxy/api = other reference app

Dependencies

• No net-new third-party dependencies were added
• If net-new third-party dependencies were added, rationale/discussion is included and 3pl-approved is set by a maintainer

---

• Tests pass (pnpm test)
• Code is formatted (pnpm run format)

[clavery]

3pl approved since it's just a net new package

[clavery]

can you put in a (minor) changeset for the package https://github.com/SalesforceCommerceCloud/b2c-developer-tooling/blob/main/CONTRIBUTING.md#changesets

that will cause the changeset workflow to bump the package in a PR and prepare for release to npm

can just say "initial release".