feat(core): Admin GUI integration

[nekolab]

Serve static assets from Canopy project

Changes include:

• new nginx config template for static file serving
• new configuration items under admin_gui_* namespace
• bundle canopy assets during the build stage
• more CORS support for Admin API
• tests and more tests

Summary

Checklist

• [ ] The Pull Request has tests
• [ ] There's an entry in the CHANGELOG
• [ ] There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• [Implement ...]

Issue reference

Fix #[issue number]