xpay: htlc_(added/removed) notifications

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