clightning: add shutdown hook for plugin

[YusukeShimizu]

Introduces a shutdown hook to the clightning plugin, ensuring that the plugin is gracefully stopped when c-lightning shuts down.

• Subscribe to the 'shutdown' event.
• Updated glightning dependency to include the 'shutdown' event. https://github.com/ElementsProject/glightning/pull/20

fixes: https://github.com/ElementsProject/peerswap/issues/307