[feature]: add timestamp to SCB

[starius]

Is your feature request related to a problem? Please describe.

This is a followup for PR #8183 "chanbackup, server, rpcserver: put close unsigned tx, remote signature and commit height to SCB" (see comment).

A timestamp in SCB file would be useful to make it more tangible for a user whether the commit_tx included into SCB is outdated or not.

Describe the solution you'd like

A backward-compatible change adding the current timestamp into chanbackup.Single type and its encoding.

chantools scbforceclose command would parse the timestamp and include it into the message it prints to the user, along with time passed since the timestamp. It would it easier to judge about backup freshness using this data.

Describe alternatives you've considered

We can put channel last-update timestamp, but it is harder to get from channeldb. Also the benefit is not clear: we are interest in the latest timestamp at which this data was still correct (i.e. no channel updates since that time).

Additional context

Unguarded use of chantools scbforceclose may result in funds loss if applied to an outdated backup. This is a last resort measure in case any other ways of recovering funds failed, including DLP protocol. We want to minimize chances of using this feature on an outdated backup file. Adding a timestamp could save funds in case the timestamp is known to be older than some channel updates.