feat: use mimir APIs to get balances

[boscohyun]

• This pull request depends on feat: introduce several balance APIs mimir#70.
• Introduce apiClient.getBalance method.
• Apply apiClient.getBalance method to "{network}/avatar/{address}" API.

[Atralupus]

How about -1? 0 really means my value is zero, but -1 is like "An error occurred."
or handling rest api response error

[boscohyun]

First of all, the existing logic is returning 0 for balance query for non-existent currencies.
If you query a balance of a currency that doesn't exist in the blockchain state,

query {
 stateQuery {
    balance(
      address: "0x6132331720a774274d6e51caead375068cc00505",
    	currency: {
        ticker: "FAKE_CRYSTAL",
        decimalPlaces: 18,
        minters: null,
      }) {
      __typename
      currency {
        ticker
      }
      quantity
    }
  }
}

and it returns "quantity": "0" like below.

{
  "data": {
    "stateQuery": {
      "balance": {
        "__typename": "FungibleAssetValueWithCurrencyType",
        "currency": {
          "ticker": "FAKE_CRYSTAL"
        },
        "quantity": "0"
      }
    }
  },
  "extensions": {}
}

So I wrote it to use 0 even if resp is null.

The cases where resp becomes null are as follows.

• CURRENCY_TICKERS[index] is null.
• CURRENCY_TICKERS[index] is not NCG, CRYSTAL or it is not match with rune and soulstone patterns.(Lib9c, Currencies.GetMinterlessCurrency(ticker))

Since CURRENCY_TICKERS is currently a fully manageable value in the 9c-board project, I think it's fine to treat quantity as 0 when resp is null unless the specification of the mimir API changes.
And after the REST API provided by mimir is switched to GraphQL, it would be nice to have a front-end handling of networking failures on the 9c-board.
Thanks!

• see also: The "stateQuery.balance" query of NineChronicles.Headless returns non-null FungibleAssetValueWithCurrencyType.