Add `services.nginx.lua.{enable,extraPackages}` to enable OpenResty's
lua-nginx-module on a stock nginx. When enabled it adds the module,
includes lua-resty-core, and wires up lua_package_path /
lua_package_cpath (and lua_ssl_trusted_certificate) from a
luajit_openresty package set built from extraPackages.
When the configured package already bundles Lua (openresty), the module
and bundled libraries are not re-added; only the search path is set up so
its own lualib stays in use.
Migrate the openresty-lua test to the new option and add an nginx-lua
test covering the stock-nginx path.
Assisted-by: Claude:claude-opus-4-8
