Exchange rates CoinMarketCap source module/CoinGecko API key support

[vbaranov]

Resolves #5600

Motivation

CoinGecko API refused requests from Blockscout without API key

Changelog

• Add CoinMarketCap Exchange rates source module
• Make CoinMarketCap source module default
• Add Environment variable EXCHANGE_RATES_SOURCE (renamed SOURCE_MODULE) with acceptable values:
  coin_market_cap - market cap from CoinMarketCap API
  coin_gecko - market cap from CoinGecko API
  token_bridge - market cap from token bridge contract
• Add EXCHANGE_RATES_COINMARKETCAP_API_KEY environment variable for setting API key for CoinMarketCap API
• Add EXCHANGE_RATES_COINGECKO_API_KEY environment variable for setting API key for CoinGecko API

CoinGecko is still the default source for exchange rates.
In order to switch to CoinMarketCap, the Blockscout instance maintainer should provide:

EXCHANGE_RATES_SOURCE=coin_market_cap
EXCHANGE_RATES_COINMARKETCAP_API_KEY=...

For CoinGecko:

EXCHANGE_RATES_SOURCE=coin_gecko # or unset EXCHANGE_RATES_SOURCE env variable
EXCHANGE_RATES_COINMARKETCAP_API_KEY=...

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[leekin96]

Is blockscout can get coin price from other side?