Changelog-Added: hsmd: Added hsmd_forget_channel to enable explicit channel deletion. ([#6987])
Motivation: Previously, a signer prematurely forgetting a channel led
to failures in unresolved channel requests. This update introduces
hsmd_forget_channel, allowing nodes to explicitly notify signers when
a channel is irrevocably resolved and can be safely forgotten. This
ensures synchronized channel cleanup between nodes and signers.
This change maintains backward and forward compatibility. Nodes
explicitly check whether a signer has `WIRE_HSMD_FORGET_CHANNEL`
capability before sending the message. Nodes without
`WIRE_HSMD_FORGET_CHANNEL` capability won't send this message. Signers
capable of handling this message but not receiving it will continue to
use conservative pruning methods.
Fixes#6987
e9ff50df96