API: Add gas fees

[ethedev]

Description

We need to add a primary call to the etherscan api with a fallback to a secondary api of public choice to get the updated gas fees value of ethereum blockchain.

/gas

{
  "slow": 124,
  "normal": 134,
  "fast": 164
}

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

Requirements

• Adding call for the specified endpoint.
• Caching the data using cron every 5 seconds to the mongo database.
• Making sure to serve the user the cached data and not have them call 3rd parties.
• On your PR include examples of what the endpoint is returning.
• Use your own mongo database for testing.
• Documenting the code.