fix sweeping chan after local force-close using cb

scenario:
- user opens a lightning channel and exports an "imported channel backup"
- user closes channel via local-force-close
  - local ctx is published, to_local output has user's funds and they are CSV-locked for days
- user restores wallet file from seed and imports channel backup
- new wallet file should be able to sweep coins from to_local output (after CSV expires)

This was not working previously, as the local_payment_basepoint was not included in the
imported channel backups, and the code was interpreting the lack of this as the channel not
having option_static_remotekey enabled. This resulted in lnutil.extract_ctn_from_tx
using an incorrect funder_payment_basepoint, and lnsweep not recognising the ctx due to
the garbage ctn value.

The imported channel backup serialisation format is slightly changed to include the
previously missing field, and its version number is bumped (0->1). We allow importing
both version 0 and version 1 backups, however v0 backups cannot handle the above
described scenario (they can only be used to request a remote-force-close).

Note that we were/are setting the missing local_payment_basepoint to the pubkey of
one of the wallet change addresses, which is bruteforceable if necessary, but I
think it is not worth the complexity to add this bruteforce logic. Also note
that the bruteforcing could only be done after the local-force-close was broadcast.

Ideally people with existing channels and already exported v0 backups should re-export
v1 backups... Not sure how to handle this.

closes https://github.com/spesmilo/electrum/issues/8516

electrum/lnsweep.py

@@ -206,6 +206,7 @@ def create_sweeptxs_for_our_ctx(
     to_local_witness_script = make_commitment_output_to_local_witness_script(
         their_revocation_pubkey, to_self_delay, our_localdelayed_pubkey).hex()
     to_local_address = redeem_script_to_address('p2wsh', to_local_witness_script)
+    to_remote_address = None
     # test if this is our_ctx
     found_to_local = bool(ctx.get_output_idxs_from_address(to_local_address))
     if not chan.is_backup():
@@ -359,6 +360,7 @@ def create_sweeptxs_for_their_ctx(
     witness_script = make_commitment_output_to_local_witness_script(
         our_revocation_pubkey, our_conf.to_self_delay, their_delayed_pubkey).hex()
     to_local_address = redeem_script_to_address('p2wsh', witness_script)
+    to_remote_address = None
     # test if this is their ctx
     found_to_local = bool(ctx.get_output_idxs_from_address(to_local_address))
     if not chan.is_backup():