introduce contract stat for dydx derivatives.

[teekenl]

• What is the purpose of this Pull Request? Introduce the contract stat for dydx perpetual derivatives. It will be listed on coingecko.com/en/derivatives and coingecko.com/en/exchanges/derivatives

• What is the related issue for this Pull Request (if this PR fixes issue, prepend with "Fixes" or "Closes")?

• [x] I have added Specs

• [x] (If implementing Market Ticker) I have verified that the volume refers to BASE

• [x] (If implementing Market Ticker) I have verified that the base and target is assigned correctly

• [x] I have implemented the trade_page_url method that links to the exchange page with the base and target passed in. If not available, enter the root domain of the exchange website.

• [x] I have verified at least ONE ticker volume matches volume shown on the trading page (use script below)

client = Cryptoexchange::Client.new
pairs = client.pairs 'exchange_name'
tickers = pairs.map do |p| client.ticker p end
sorted_tickers = tickers.sort_by do |t| t.volume end.reverse

[teekenl]

@ElvisLYC are you able to take a look at this PR. for spot trading, do we skip and return nothing for contract_stat ?

[ElvisLYC]

@teekenl my suggestion is:-

1) Before deployment, double check with the ticket assignor that all tickers listed here https://api.dydx.exchange/v1/stats/markets is referring to perpetual tickers (because it is stated as type spot in the api)

2)you don't need dydx and dydx_perpetual market, choose only one. referring to my comment above

[ElvisLYC]

LGTM