feat(gui): added a new feature to support multiple domains for gui.

[raoxiaoyan]

Summary

1. By default, set the origin retrieved from the request header or '*'.
2. If the admin_gui_url has more than one domain, the origin from the request header must match one of the admin_gui_url. Otherwise, It can't access the Admin API via the Kong Manager.
   For example:
   admin_gui_url=http://example.com,http://km.konghq.com/manager
   These addresses access Kong Manager works well. http://example.com and http://km.konghq.com/manager.
   Others can't.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

KM-516

[sumimakito]

Thanks for the effort. Let's decide the optimal way to implement this feature before going deep into this.