Generate proposal for "ionic swap"

### Request

```
{
  "jsonrpc": "2.0",
  "id": "0",
  "method": "ionic_swap_generate_proposal",
  "params": {
    "proposal": {
      "to_finalizer": [{
        "asset_id": "e03a140b8447d2895290022b25c06bdabea514e2475ae56ce5bcbc554ab9865c",
        "amount": 2000000000000
      }],
      "to_initiator": [{
        "asset_id": "d6329b5b1f7c0805b5c345f4957554002a2f557845f64d7645dae0e051a6498a",
        "amount": 10000000000000
      }],
      "mixins": 10,
      "fee_paid_by_a": 10000000000,
      "expiration_time": 0
    },
    "destination_address": "ZxDAcbaxXkyWRgYbeARBpngfmFat5TjDjjQA5NAbouB9eytwGWJqA5shAVYeCAHWPo254DF2o2X1td79PNvRr2Yc1b9Ep67ff"
  }
}
```

**to_finalizer** - assets meant for finalizer(party that receives proposal)

**to_initiator** - assets that initiator (party that initiated ionic_swap operation and creating proposal) expect to get in exchange, those assets expected to be fulfilled by finalizer

**fee_paid_by_a**- specify which amount of transaction fee is paid by initiator (the rest if needed supposed to be added by finalizer)

**expiration_time** - proposal expiration time, if 0 - then will be automatically set to 1 hour from current network time.

### Response

```
{
  "id": "0",
  "jsonrpc": "2.0",
  "result": {
    "hex_raw_proposal": "[encoded data, 12280 characters]"
  }
}
```
