Add exchange rates

[chris124567]

• Create interface for exchange rate APIs
• Add CoinGecko and Kraken implementors
• Add implementor that can average exchange rates from different sources
• Add API endpoint for exchange rate in USD

The implementors just retrieve the exchange rate every x seconds and cache the results. This way the number of requests doesn't scale with the number of users and we won't exceed rate limits.

[chris124567]

This seems pretty extensible. We could also respond with the rates for all the currencies at once if you think that'd be better.

[n8maninger]

I think it's fine for our API to return a single rate per request. However, a ticker per source per pair is less than ideal. For example, with Coingecko, you can get all the currencies you need at once:

https://api.coingecko.com/api/v3/simple/price?ids=bitcoin%2Csiacoin%2Csiaprime-coin%2Cbitcoin&vs_currencies=btc%2Cusd%2Ccad%2Ceur%2Cgbp%2Cjpy%2Caus%2Crub%2Ccny%2Ceth&include_market_cap=false&include_24hr_vol=false&include_24hr_change=false&include_last_updated_at=false

Ideally, you could refresh all the sources at the same rate and save on a lot of unnecessary tickers.