Plugins: 'channel_closed' notification type

[NishantBansal2003]

Ref: #3207 #3662

PR Description

This adds a new notification type: channel_closed.

This notification is emitted if a channel has been successfully closed with a peer and sends the peer's ID and the closing transaction ID.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.

[NishantBansal2003]

Although the channel closure state can currently be notified via channel_state_changed, it would be more symmetrical to have a dedicated channel_closed event analogous to the channel_opened event.