fix: Handle CORS for OS proxy endpoints

[DafyddLlyr]

What does this PR do?

• Fix OS proxy endpoints to allow correct list of origins defined in MAP_ALLOWLIST
• Pull this function out of the proxy, and place it into a middleware layer

To test

• Works localhost → localhost
• Works localhost → Pizza
• Works from oslmap.netlify.app → Staging
  • https://oslmap.netlify.app/#!/components/vanilla/mymap---proxy shows OS tiles via proxy
• Works from <PR_NUMBER>--oslmap.netlify.app → Staging
  • https://deploy-preview-427--oslmap.netlify.app/#!/components/vanilla/mymap---proxy shows OS tiles via proxy

[github-actions]

Removed vultr server and associated DNS entries

[Mike-Heneghan]

What does this PR do?

• Fix OS proxy endpoints to allow correct list of origins defined in MAP_ALLOWLIST
• Pull this function out of the proxy, and place it into a middleware layer

To test

• Works localhost → localhost
• Works localhost → Pizza
• Works from oslmap.netlify.app → Staging
• Works from <PR_NUMBER>--oslmap.netlify.app → Staging

Hey @DafyddLlyr 👋

I'm keen to test and have a look at this PR although I'm struggling a little to figure out the oslmap steps 🤔

Would you be able to add a little more context or maybe point me at some info which might put me on the right path?

[Mike-Heneghan]

The code change here seems like a really good solution.

Although I'm not confident about ✅ as I haven't tested this.

[jessicamcinchak]

Working for me - thanks for helping troubleshoot this one!

@Mike-Heneghan testing instructions found in description of this first-attempt PR #2672