feat: Exchange

[aleksasiriski]

Adds a new endpoint e.g. /exchange?from=BTC&to=RSD&amount=1 that uses the configured base currency (EUR) to achieve the conversion. The conversion rates for the base currency are pulled from all engines and stored in cache db with the default TTL of 24h.

The response looks like this:

{
  "version": "dev",
  "duration": 501,
  "base": "EUR",
  "from": "BTC",
  "to": "RSD",
  "amount": 1,
  "result": 6887344.831246245
}

• Configurable TTL
• Configurable base currency
• Configurable enabled engines
• Generated exchanger code like how it's done for searcher & suggester (enginer).