feat: add USD price feed support for PriceFeedOracle

[rackstar]

Description

Closes #1260

These changes adds support for USD-based price feeds to be converted to ETH rate in PriceFeedOracle.

Context:

We want to onboard cbBTC as a new Pool asset but currently cbBTC does not have an ETH price feed only cbBTC-USD priceFeed. These PriceFeedOracle changes are required so that we can onboard cbBTC as a coverAsset.

Testing

• unit tests
• fork tests

Checklist

• [x] Performed a self-review of my own code
• [x] Made corresponding changes to the documentation

[rackstar]

closing in favour of: https://github.com/NexusMutual/smart-contracts/pull/1275