auth/reverseproxy: Add support for full name (#20776)

This adds support for getting the user's full name from the reverse
proxy in addition to username and email.

Tested locally with caddy serving as reverse proxy with Tailscale
authentication.

Signed-off-by: Will Norris <will@tailscale.com>

Signed-off-by: Will Norris <will@tailscale.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

custom/conf/app.example.ini

@@ -377,9 +377,10 @@ INTERNAL_TOKEN=
 ;; Name of cookie used to store authentication information.
 ;COOKIE_REMEMBER_NAME = gitea_incredible
 ;;
-;; Reverse proxy authentication header name of user name and email
+;; Reverse proxy authentication header name of user name, email, and full name
 ;REVERSE_PROXY_AUTHENTICATION_USER = X-WEBAUTH-USER
 ;REVERSE_PROXY_AUTHENTICATION_EMAIL = X-WEBAUTH-EMAIL
+;REVERSE_PROXY_AUTHENTICATION_FULL_NAME = X-WEBAUTH-FULLNAME
 ;;
 ;; Interpret X-Forwarded-For header or the X-Real-IP header and set this as the remote IP for the request
 ;REVERSE_PROXY_LIMIT = 1
@@ -694,6 +695,7 @@ ROUTER = console
 ;ENABLE_REVERSE_PROXY_AUTHENTICATION = false
 ;ENABLE_REVERSE_PROXY_AUTO_REGISTRATION = false
 ;ENABLE_REVERSE_PROXY_EMAIL = false
+;ENABLE_REVERSE_PROXY_FULL_NAME = false
 ;;
 ;; Enable captcha validation for registration
 ;ENABLE_CAPTCHA = false