Rest endpoint to load by projectId

[andrasbacsai]

Description

1. What is this PR about (link the issue and add a short description)
   Add REST endpoint to get the latest buildId based on projectId:
   /rest/buildId/<projectId>

Should return buildId or null (which means the project is not published yet).

It is required for the new CLI.

Steps for reproduction

1. click button
2. expect xyz

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 5de6)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env.example and the builder/env-check.js if mandatory

[vercel]

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Name	Status	Preview	Updated (UTC)
webstudio-builder	✅ Ready (Inspect)	Visit Preview	Jun 28, 2023 10:52am

[istarkov]

plz reuse same code

[istarkov]

Endpoint name looks strange, we dont return project, plz change