Add dynamic token support to onlyswaps-client

[azixus]

[...] we also added a /currencies endpoint that could be useful to add here! it's very simple, just a static GET endpoint with no params!
Originally posted by @CluEleSsUK in #180 (comment)

Instead of having to statically specify which currencies are currently supported by which chain, it would be a nice addition to fetch the configuration directly from the /currencies endpoint:

> curl https://fees.onlyswaps.dcipher.network/currencies | jq
{
  "currencies": [
    {
      "symbol": "RUSD",
      "chain_id": 43113,
      "address": "0x908e1d85604e0e9e703d52d18f3f3f604fe7bb1b",
      "decimals": 18
    },
    {
      "symbol": "RUSD",
      "chain_id": 43114,
      "address": "0x1b0f6cf6f3185872a581bd2b5a738eb52ccd4d76",
      "decimals": 18
    },
    {
      "symbol": "RUSD",
      "chain_id": 84532,
      "address": "0x908e1d85604e0e9e703d52d18f3f3f604fe7bb1b",
      "decimals": 18
    },
    {
      "symbol": "RUSD",
      "chain_id": 8453,
      "address": "0x1b0f6cf6f3185872a581bd2b5a738eb52ccd4d76",
      "decimals": 18
    },
    {
      "symbol": "USDT",
      "chain_id": 43114,
      "address": "0x9702230a8ea53601f5cd2dc00fdbc13d4df4a8c7",
      "decimals": 6
    },
    {
      "symbol": "USDT",
      "chain_id": 8453,
      "address": "0xfde4c96c8593536e31f229ea8f37b2ada2699bb2",
      "decimals": 6
    }
  ]
}