This feature was removed by #22219 to avoid possible CSRF attack.
This PR takes reverseproxy auth for API back but with default disabled.
To prevent possbile CSRF attack, the responsibility will be the
reverseproxy but not Gitea itself.
For those want to enable this `ENABLE_REVERSE_PROXY_AUTHENTICATION_API`,
they should know what they are doing.
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Co-authored-by: Giteabot <teabot@gitea.io>
