feat(gui): added a new feature to support proxy Admin API via gui.

[raoxiaoyan]

Summary

1. By default, if the admin_gui_api_url was not set to a value, The admin API is proxy by Kong Gateway.
2. If the admin_gui_api_url was set to a special value. The Admin API is proxy by Kong Manager.

admin_gui_api_url = http://localhost:8001

Checklist

• [x] The Pull Request has tests
• [x] 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-478