Design and Build Pricing Service

[adamgall]

We need to build a backend price supplier API.

Here are some requirements:

• It should include IP-based rate limiting.
• It should include CORS-based limiting, same origin.
• It uses CoinGecko's Free Tier API key, which has a limit of 30 requests per minute.
• Service is stateless with regards to Safes, it doesn't know anything about Safe addresses.
• Service accepts a list of token addresses (and/or ETH), and responds back with USD value for each token address (and/or ETH).

I see the hard part being satisfying these three things at the same time. As long as number 3 is a should, not a must, we'll be gucci:

1. Service must always respond to a request in a timely manner (no slower than however much longer a CoinGecko request takes, plus some time for our service to build a response).
2. Service must, globally, never hit the CoinGecko API more than 30 times in a 1 minute window.
3. Service should not return stale price data ("staleness" to be tweaked later, lets call it 30 mins for now).

[adamgall]

Closing in favor of #1362