Add `channel_keys_id` to `SpendableOutputDescriptor::StaticOutput`

Add `channel_keys_id` to `SpendableOutputDescriptor::StaticOutput`

In 7f0fd868ad4e8072440f1eb79e78894de1629157, `channel_keys_id` was
added as an argument to `SignerProvider::get_destination_script`,
allowing implementors to generate a new script for each channel.

This is great, however users then have no way to re-derive the
corresponding private key when they ultimately receive a
`SpendableOutputDescriptor::StaticOutput`. Instead, they have to
track all the addresses as they derive them separately. In many
cases this is fine, but we should support both deployments, which
we do here by simply including the missing `channel_keys_id` for
the user.