xpay: htlc_(added/removed) notifications

[michael1011]

xpay (and maybe other pay plugins) should emit htlc_added and htlc_removed notifications. This is particularly useful for xpay to get the information needed to create layers based on data of your own payments. These notifications should include the full route of the HTLC

Reference: this Twitter thread https://x.com/rusty_twit/status/1873853663757955137

[kilrau]

@rustyrussell pretty please, we need this - it'll let us fix the remaining issues xpay has 🙏