Add price feed pallet

[mattsse]

Changes

Blocked by #39 because the substate-v3 chainlink feed depends on a newer substrate rococo revision

• Adds skeleton of price-feed pallet
• Integrates the chainlink-feed pallet via FeedOracle
• Adds AssetPricePair type to represent a base/quote price pair
• Adds PirceFeed trait to fetch asset pairs

The chainlink pallet is abstracted via the FeedOracle trait, the PINT price-feed pallet will be an additional abstraction on top of it. While the design for the price-feed isn't final, I believe we probably will need some storage items, therefor the price-feed will be a pallet instead of an additional trait that is implemented for FeedOracle

The PriceFeed trait is the intended abstraction on top the price feed pallet
New prices are inserting into the designated feeds via the existing chainlink feed pallet; At this point it is assumed that all feeds, including the PINT price feed will use a common base currency (DOT).
The price feed pallet is configured with a Percision which is applied to all price feeds, since feeds can have different decimals, a conversion will be required.

Update:

• FixedU128 was chosen as type to represent price ratios.
• The admin needs to create a mapping between assetId and feedId.

Tests

cargo test

Issues

• closes Integrate oracle pallet and add price feed #30

[willemolding]

I'm having trouble building this locally and also I think there is a cargo fmt error.

New additions look great. Some more test would be good. The feed_creation_should_work is just a smoke test, we should probably at least test calling the get_price implementation. It would also be great to have test for the AssetPricePair methods.

[willemolding]

Looks great! Happy to merge once you have sorted out the dependency issues