[feature]: create an initial basic version of the PriceOracle RFQ service

[ffranr]

Parent issue: #683

The PriceOracle is a new RFQ subsystem service for retrieving the exchange rate between BTC and a counter currency. The service is queried by the RFQ Negotiator service.

[ffranr]

The unknowns I'd like to solve are:

1. How do I get from an asset ID/asset group to some bid/ask on a site like blockchain.com
2. How do I handle niche assets where I can't find a bid/ask on the internet.

Do we need meta data in the asset to the effect of "price this asset as this fiat currency"?

Any ideas @Roasbeef @jharveyb @dstadulis ?

[Roasbeef]

How do I get from an asset ID/asset group to some bid/ask on a site like blockchain.com

From the code's PoV, it's an abstract interface it can use to obtain price data. It isn't aware of where it's getting that data from, just that it can map a (asset, volume) pair into an acceptable price.

The actual implementation can source data from pretty much anywhere. We'll fashion a standardized JSON/gRPC interface similar to the feeurl setting we have. A daemon won't ever need to source prices of an asset that it doesn't hold itself.

How do I handle niche assets where I can't find a bid/ask on the internet.

I don't think this is a concern. See above.

The code can be written in an abstracted style, trusting that if a daemon has an asset, then it'll be able to obtain a relevant price for it. If not, then there'll be an error scenario where a user may need to manually update a config, or run a slim proxy to be able to obtain a quote.

More concretely, I think this would take the form of a PirceOracle gRPC service that tapd becomes a client of. We hit this endpoint whenever we need to obtain a price that isn't cache otherwise. This can also instead be configured as a normal REST endpoint returning JSON in w/e format.

Do we need meta data in the asset to the effect of "price this asset as this fiat currency"?

What would that look like in practice? I think, no? The logic of an oracle implementation itself can be very dynamic, and isn't really restricted by much. You can think of it just like fee estimation in a sense.