Gas fee paid in ZETA token

[brewmaster012]

When a Send state variable is created, gas fee on receiver chain needs to be computed and converted into ZETA token. Then it's subtracted from the ZETA burnt, which will be the mint amount.

Right now we dont have price oracle of ZETA/ETH. We use 1.0 for testing.

The price oracle should come from 3rd party service, or from on-chain pool ration (sushi, uniswap).

This PR also creates gas estimation for the UI: example JSON query:

Should use the median price. The unit is wei (10^-18 ETH).

{
  "GasPrice": [
    {
      "creator": "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9",
      "index": "BSC",
      "chain": "BSC",
      "prices": {
        "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9": "10000000000",
        "meta1ftcxyydkksy2g98az595zg4v6tg96d8yx4clay": "10000000000"
      },
      "median": "10000000000",
      "blockNum": {
        "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9": "13811498",
        "meta1ftcxyydkksy2g98az595zg4v6tg96d8yx4clay": "13811499"
      },
      "medianBlock": "13811498"
    },
    {
      "creator": "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9",
      "index": "ETH",
      "chain": "ETH",
      "prices": {
        "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9": "1940000300",
        "meta1ftcxyydkksy2g98az595zg4v6tg96d8yx4clay": "1940000300"
      },
      "median": "1940000300",
      "blockNum": {
        "meta13g3pgefm8alqv9244vdnph50tralmlnpvuypt9": "5788559",
        "meta1ftcxyydkksy2g98az595zg4v6tg96d8yx4clay": "5788560"
      },
      "medianBlock": "5788560"
    }
  ],
  "pagination": {
    "next_key": null,
    "total": "2"
  }
}

====
closes #33
closes #28