API: Add YAM avg price

[ethedev]

Description

We need to add a primary call to the coingecko api with a fallback to a secondary api of public choice, to get the yam 30 days avg price.

• Coingecko: https://www.coingecko.com/en/coins/yam
• To calculate it, we need to save 2 values as follows:
  1. 30d should be the last 1 month full number of days, value saved every day for the yam price, averaging it over the days to get the value (from 1st to last day of the month).
  2. now should be the last 30 days from today, value is also saved every day (example today is the 14th of the month, we average it as well over the days from: 14th day of last month - to date, this value is more up to date and not a fixed value).

/30avg

{
	"30d": "2.00",
	"now": "1.63",
}

Follow the .env.example to add or use secrets if needed.

Requirements

• Adding call for the specified endpoint.
• Caching the data using cron to the mongo database.
• Use your own mongo database for testing.
• On your PR include examples of what the endpoint is returning.
• Documenting the code.