Add notifications for htlc expiry

[JssDWt]

Whenever a htlc is about to expire, and the client is subscribed to notifications, notify the webhook with a htlc expiry notification.

[roeierez]

I do see a pending PR in greenlight for web hooks I think the intention in this PR is to notify before force close exactly for this use case

[JssDWt]

I do see a pending PR in greenlight for web hooks I think the intention in this PR is to notify before force close exactly for this use case

Ah good point

[JssDWt]

Closing this for now, because greenlight webhooks are suitable for most usecases.

[kingonly]

Let's keep it open with a reference to the GL PR

[JssDWt]

There is this PR for webhook support, but it looks like it's not aimed at specific usecases yet: https://github.com/Blockstream/greenlight/pull/334