Chainlink oracles

[MathisGD]

Handles notably these cases (let's say that our pair is A/B):

• A/B is a feed (typically, stETH/ETH).
• B/A is a feed (typically, USDC/ETH).
• A/C and B/C are feeds (typically, stETH/ETH and USDC/ETH).
• A/C, C/D and B/D are feeds (typically, WBTC/BTC, BTC/USD, USDC/USD).
• A/D, and B/C, C/D are feeds (typically, USDC/USD, WBTC/BTC, BTC/USD).
• A/C, C/D and B/E, E/D are feeds.
• A/C and C/B are feeds (typically, WBTC/BTC and BTC/ETH).
• and some other cases...

We are missing the feature of oracles that price tokens that have an exchange rate against a well-known token, like sDAI. This is implemented in #7 (not ready for onchain team review yet).