Option to regenerate `SpendableOutputs`

[TheBlueMatt]

We should really have an option to regenerate these given historical transaction data. While we require users to store these, users have occasionally not done so, and regenerating them should be pretty doable.

[tlulu]

I would like to take this on

[TheBlueMatt]

Go for it! From a quick glance it looks like we basically always generate these via MaturingOutput via ChannelMonitor::is_paying_spendable_output, so it really shouldn't be a bad refactor to refactor that function to also allow a user to submit a transaction and get the SpendableOutputDescriptor(s) directoy for transactions >= 5 blocks ago.