use CoinGecko instead of CoinMarketcap for exchange rates

CoinMarketcap deprecated and took offline its v1 API. They have
v2 API which has free plan and requires API keys. But as part of
https://github.com/poanetwork/blockscout/issues/234 we alresy
implemented fetching from CoinGecko.

Now, coin id should be set for CoinGecko for the required coin.
```
config :explorer, Explorer.ExchangeRates.Source.CoinGecko,
  coin_id: System.get_env("COIN_GECKO_ID", "poa-network")
```
Or the `COIN_GECKO_ID` env var should be set. For example, COIN_GECKO_ID=poa-network

apps/explorer/config/config.exs

@@ -34,6 +34,8 @@ config :explorer, Explorer.Chain.Cache.BlockNumber, enabled: true
 config :explorer, Explorer.ExchangeRates.Source.CoinMarketCap,
   pages: String.to_integer(System.get_env("COINMARKETCAP_PAGES") || "10")
 
+config :explorer, Explorer.ExchangeRates.Source.CoinGecko, coin_id: System.get_env("COIN_GECKO_ID", "poa-network")
+
 balances_update_interval =
   if System.get_env("ADDRESS_WITH_BALANCES_UPDATE_INTERVAL") do
     case Integer.parse(System.get_env("ADDRESS_WITH_BALANCES_UPDATE_INTERVAL")) do

apps/explorer/lib/explorer/chain.ex

@@ -2717,7 +2717,7 @@ defmodule Explorer.Chain do
   end
 
   defp supply_module do
-    Application.get_env(:explorer, :supply, Explorer.Chain.Supply.CoinMarketCap)
+    Application.get_env(:explorer, :supply, Explorer.Chain.Supply.ExchangeRate)
   end
 
   @doc """

apps/explorer/lib/explorer/chain/supply/exchange_rate.ex

@@ -1,6 +1,6 @@
-defmodule Explorer.Chain.Supply.CoinMarketCap do
+defmodule Explorer.Chain.Supply.ExchangeRate do
   @moduledoc """
-  Defines the supply API for calculating supply for coins from coinmarketcap.
+  Defines the supply API for calculating supply for coins from exchange_rate..
   """
 
   use Explorer.Chain.Supply

apps/explorer/lib/explorer/exchange_rates/source.ex

@@ -83,7 +83,7 @@ defmodule Explorer.ExchangeRates.Source do
 
   @spec exchange_rates_source() :: module()
   defp exchange_rates_source do
-    config(:source) || Explorer.ExchangeRates.Source.CoinMarketCap
+    config(:source) || Explorer.ExchangeRates.Source.CoinGecko
   end
 
   @spec config(atom()) :: term

apps/explorer/lib/explorer/exchange_rates/source/coin_gecko.ex

@@ -15,39 +15,45 @@ defmodule Explorer.ExchangeRates.Source.CoinGecko do
     {:ok, price} = get_btc_price()
     btc_price = to_decimal(price)
 
-    for item <- decode_json(data),
-        not is_nil(item["total_supply"]) and not is_nil(item["current_price"]) do
-      {:ok, last_updated, 0} = DateTime.from_iso8601(item["last_updated"])
+    json_data = decode_json(data)
 
-      current_price = to_decimal(item["current_price"])
+    market_data = json_data["market_data"]
+    {:ok, last_updated, 0} = DateTime.from_iso8601(market_data["last_updated"])
 
-      id = item["id"]
+    current_price = to_decimal(market_data["current_price"]["usd"])
+
+    id = json_data["id"]
     btc_value = if id != "btc", do: Decimal.div(current_price, btc_price), else: 1
 
+    [
       %Token{
-        available_supply: to_decimal(item["total_supply"]),
-        total_supply: to_decimal(item["total_supply"]),
+        available_supply: to_decimal(market_data["circulating_supply"]),
+        total_supply: to_decimal(market_data["total_supply"]),
         btc_value: btc_value,
-        id: id,
+        id: json_data["id"],
         last_updated: last_updated,
-        market_cap_usd: to_decimal(item["market_cap"]),
-        name: item["name"],
-        symbol: item["symbol"],
+        market_cap_usd: to_decimal(market_data["market_cap"]["usd"]),
+        name: json_data["name"],
+        symbol: String.upcase(json_data["symbol"]),
         usd_value: current_price,
-        volume_24h_usd: to_decimal(item["total_volume"])
+        volume_24h_usd: to_decimal(market_data["total_volume"]["usd"])
       }
-    end
+    ]
   end
 
   @impl Source
-  def source_url(currency \\ "usd") do
-    "#{base_url()}/coins/markets?vs_currency=#{currency}"
+  def source_url do
+    "#{base_url()}/coins/#{coin_id()}"
   end
 
   defp base_url do
     config(:base_url) || "https://api.coingecko.com/api/v3"
   end
 
+  defp coin_id do
+    Application.get_env(:explorer, __MODULE__)[:coin_id]
+  end
+
   defp get_btc_price(currency \\ "usd") do
     url = "#{base_url()}/exchange_rates"