use 32-byte channel ids instead of short channel ids

The `swap_in_request` and `swap_out_request` messages currently pass a `scid` (short channel id) parameter. It would be easier to process and more consistent with existing Lightning messages if instead these messages used a 32-byte `channel_id` to identify the channel being requested.

Any channels used for PeerSwap should have a corresponding confirmed on-chain opening tx and thus a permanent `channel_id` defined. Current Lightning messages use the 32-byte `channel_id` to identify channels. A `short_channel_id` is only needed for routing, which is not relevant for PeerSwap.

In particular, Eclair watches for payments based on their `channel_id` and using `short_channel_id` in PeerSwap messages requires an unnecessary lookup or additional parameter passing. This may be true for other implementations also.