Add example basic price oracle service

[ffranr]

Closes #995

This PR introduces a basic price oracle service example and ensures its compilation during our CI checks.

[Roasbeef]

Based on this, I think we need to update the proto comments and other godocs re the "rate tick". Initially it started out at one ten-thousandth, but during the final portion of dev, switching over to units per mSAT made more sense (as that's the smallest amount we can send).

[ffranr]

I've updated this comment (and the one below) to state "42,000 asset units per mSAT."

[Roasbeef]

Can we add more godoc details to the diff of the two transaction types? Perhaps with a brief walkthrough example? So something like: Alice has X and wants to pay an invoice, she queries the oracle for a quote with transactionType Z.

On second thought, this sounds like something we should add as normal mark down docs, and also in the builder's guide.

[ffranr]

In the latest commits I've included an example in the doc for QueryRateTick.

[Roasbeef]

Is my understanding correct that PaymentAssetis the asset that'll be sent outgoing? Though if I think about the send vs recv flows, that means in the send flow the PaymentAsset is BTC, but in the recv flow (receiving some other asset) it'll be that asset?

Or is it that this is the asset the penultimate hop will be receiving?

[ffranr]

I conceptualize the price oracle service as follows: it does not know on whether an asset is inbound or outbound on a channel; its primary function is to return a price. This applies regardless of whether the node is attempting to formulate a rate tick hint price or a quote accept rate tick.

Hence, it is the client's responsibility to select the SubjectAsset and PaymentAsset according to their needs. The TransactionType determines the trade side the client is interested in.

The price oracle is currently called in two places:

taproot-assets/rfq/oracle.go

Lines 310 to 326 in eeca76c

	paymentAssetId := make([]byte, 32)
	req := &oraclerpc.QueryRateTickRequest{
	TransactionType: oraclerpc.TransactionType_PURCHASE,
	SubjectAsset: &oraclerpc.AssetSpecifier{
	Id: &oraclerpc.AssetSpecifier_AssetId{
	AssetId: subjectAssetId,
	},
	},
	SubjectAssetMaxAmount: maxAssetAmount,
	PaymentAsset: &oraclerpc.AssetSpecifier{
	Id: &oraclerpc.AssetSpecifier_AssetId{
	AssetId: paymentAssetId,
	},
	},
	RateTickHint: nil,
	}

taproot-assets/rfq/oracle.go

Lines 228 to 257 in eeca76c

	paymentAssetId := make([]byte, 32)
	// Construct the RPC rate tick hint.
	var rateTickHint *oraclerpc.RateTick
	if bidPrice != nil {
	// Compute an expiry time using the default expiry delay.
	expiryTimestamp := uint64(time.Now().Unix()) +
	defaultRateTickExpirySeconds
	rateTickHint = &oraclerpc.RateTick{
	Rate: uint64(*bidPrice),
	ExpiryTimestamp: expiryTimestamp,
	}
	}
	req := &oraclerpc.QueryRateTickRequest{
	TransactionType: oraclerpc.TransactionType_SALE,
	SubjectAsset: &oraclerpc.AssetSpecifier{
	Id: &oraclerpc.AssetSpecifier_AssetId{
	AssetId: subjectAssetId,
	},
	},
	SubjectAssetMaxAmount: assetAmount,
	PaymentAsset: &oraclerpc.AssetSpecifier{
	Id: &oraclerpc.AssetSpecifier_AssetId{
	AssetId: paymentAssetId,
	},
	},
	RateTickHint: rateTickHint,
	}

At both call sites the payment asset is BTC and it's the transaction type that varies.

I've tried to develop a basic price oracle in a generalized manner, such that it returns a price without any awareness of asset channels. I think that makes sense for light tap clients but maybe not for edge nodes?

[Roasbeef]

LGTM 🦄

[Roasbeef]

I think it would be good to further lift this into a high level markdown doc (along with how to set one up, the flags, etc). Don't think we need to block merging this based on that though.

[ffranr]

I think I've mentioned this to @Liongrass .