Only the fee invoice amount is added to the ListPeers RPC paid fee

Only the fee invoice at the time of swapout is recorded in the paid fee displayed in ListPeers.
* https://github.com/ElementsProject/peerswap/blob/master/peerswaprpc/peerswaprpc.proto#L12

Related: https://discord.com/channels/905126649224388629/905127419466350613/1191356265549533284.

### Example Situation
Swap in and swap out once each from initiator node.
* initiator_node_id: 03926095277db2522ed467ab2e7a108cd771a994e39ef3945175fe1a24d4aba5a2
* responder_node_id:031872bccffbff6203d68bcaac7d2a088bc13e35026b31fdabe32a6d272397fe1a

```sh
peerswap1 listswaps
{
"swaps": [
{
"id": "b963683c9ea47dbf23b74e35c131f262d6673d62739946b367eda0c8fbf6bf1c",
"created_at": "1704244509",
"asset": "btc",
"type": "swap-out",
"role": "sender",
"state": "State_ClaimedPreimage",
"initiator_node_id": "03926095277db2522ed467ab2e7a108cd771a994e39ef3945175fe1a24d4aba5a2",
"peer_node_id": "031872bccffbff6203d68bcaac7d2a088bc13e35026b31fdabe32a6d272397fe1a",
"amount": "10000000",
"channel_id": "110:1:1",
"opening_tx_id": "10912f55649640f8f829ddf029a0ad9462eb2c65ef335c2945c64d5c16887af0",
"claim_tx_id": "f4965e25d53f509464153f088594db67c23d080949ba38732a165a10b1b06dbf",
"cancel_message": "",
"lnd_chan_id": "120946279120897"
},
{
"id": "25ec85b3e22af44c1eeb862ae664949a11bb0ee49919ef0058352cceb75898cf",
"created_at": "1704244534",
"asset": "btc",
"type": "swap-in",
"role": "sender",
"state": "State_ClaimedPreimage",
"initiator_node_id": "03926095277db2522ed467ab2e7a108cd771a994e39ef3945175fe1a24d4aba5a2",
"peer_node_id": "031872bccffbff6203d68bcaac7d2a088bc13e35026b31fdabe32a6d272397fe1a",
"amount": "1000000",
"channel_id": "110:1:1",
"opening_tx_id": "8a7bbaed84cf0250f5051df15161e34ef14fcf2732ea00982628cddbf46ac1bf",
"claim_tx_id": "",
"cancel_message": "",
"lnd_chan_id": "120946279120897"
}
]
}
```

#### Initiator
The paid fee is the amount of the fee invoice.

```sh
./bin/pscli peerswap1 listpeers
{
"peers": [
{
"node_id": "031872bccffbff6203d68bcaac7d2a088bc13e35026b31fdabe32a6d272397fe1a",
"swaps_allowed": true,
"supported_assets": [
"btc",
"lbtc"
],
"channels": [
{
"channel_id": "120946279120897",
"local_balance": "90979030",
"remote_balance": "9017500",
"active": true
}
],
"as_sender": {
"swaps_out": "1",
"swaps_in": "1",
"sats_out": "10000000",
"sats_in": "1000000"
},
"as_receiver": {
"swaps_out": "0",
"swaps_in": "0",
"sats_out": "0",
"sats_in": "0"
},
"paid_fee": "17500"
}
]
}
```

#### Responder
The paid fee is 0.
```sh
peerswap2 listpeers
{
"peers": [
{
"node_id": "03926095277db2522ed467ab2e7a108cd771a994e39ef3945175fe1a24d4aba5a2",
"swaps_allowed": true,
"supported_assets": [
"btc",
"lbtc"
],
"channels": [
{
"channel_id": "120946279120897",
"local_balance": "9017500",
"remote_balance": "90979030",
"active": true
}
],
"as_sender": {
"swaps_out": "0",
"swaps_in": "0",
"sats_out": "0",
"sats_in": "0"
},
"as_receiver": {
"swaps_out": "1",
"swaps_in": "1",
"sats_out": "10000000",
"sats_in": "1000000"
},
"paid_fee": "0"
}
]
}
```

### Idea for improvement
It would be good if the cost of onchain(opening tx/claim tx) incurred is additionally accounted for.