Add NGINX config snippets

debian/ula-nginx-authenticator.install

-usr/bin/nginx-authenticator
+debian/ula_auth.conf etc/nginx/conf.d
+debian/ula_auth.snippet etc/nginx/snippets
+

debian/ula_auth.conf

+upstream ula_auth {
+  server unix:/run/nginx-authenticator/socket;
+}

debian/ula_auth.snippet

+location = /__auth {
+  proxy_pass http://nginx_auth;
+  proxy_pass_request_body off;
+  proxy_set_header Content-Length "";
+}
+