Support Vouch Proxy as an opional SSO login gateway

[blackandred]

Idea:
To allow hiding administrative services behind a login page.

Technically:
The proxy could be optionally installable via Harbor Snippet Cooperative. But the support must be anyway in the NGINX template.

https://github.com/vouch/vouch-proxy
https://developer.okta.com/blog/2018/08/28/nginx-auth-request