Add new `dcr-usdt` market sources - Huobi, Kucoin and Coinex

[ukane-philemon]

• API Docs for Coinex: https://docs.coinex.com/api/v2/spot/market/http/list-market, https://docs.coinex.com/api/v2/spot/market/http/list-market-depth, https://docs.coinex.com/api/v2/spot/market/http/list-market-kline

• API Docs for Kucoin: https://www.kucoin.com/docs/rest/spot-trading/market-data/introduction. All the required API endpoints are under the Market data section

• API docs for Huobi: https://www.htx.com/en-us/opend/newApiPages/?id=648. All the required API endpoints are under the Market data section

[ukane-philemon]

Requires: #2002 (comment)