Implement logout redirection for reverse proxy auth setups (#36085)

When authentication is handled externally by a reverse proxy SSO
provider, users can be redirected to an external logout URL or relative
path defined on the reverse proxy.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/auth/auth.go

@@ -493,12 +493,17 @@ func SignOut(ctx *context.Context) {
 }
 
 func buildSignOutRedirectURL(ctx *context.Context) string {
-	// TODO: can also support REVERSE_PROXY_AUTHENTICATION logout URL in the future
 	if ctx.Doer != nil && ctx.Doer.LoginType == auth.OAuth2 {
 		if s := buildOIDCEndSessionURL(ctx, ctx.Doer); s != "" {
 			return s
 		}
 	}
+
+	// The assumption is: if reverse proxy auth is enabled, then the users should only sign-in via reverse proxy auth.
+	// TODO: in the future, if we need to distinguish different sign-in methods, we need to save the sign-in method in session and check here
+	if setting.Service.EnableReverseProxyAuth && setting.ReverseProxyLogoutRedirect != "" {
+		return setting.ReverseProxyLogoutRedirect
+	}
 	return setting.AppSubURL + "/"
 }