add a mempool client for fee estimation

chain/fee_estimator.go

@@ -0,0 +1,21 @@
+package chain
+
+import "context"
+
+type FeeStrategy int
+
+const (
+	FeeStrategyFastest  FeeStrategy = 0
+	FeeStrategyHalfHour FeeStrategy = 1
+	FeeStrategyHour     FeeStrategy = 2
+	FeeStrategyEconomy  FeeStrategy = 3
+	FeeStrategyMinimum  FeeStrategy = 4
+)
+
+type FeeEstimation struct {
+	SatPerVByte float64
+}
+
+type FeeEstimator interface {
+	EstimateFeeRate(context.Context, FeeStrategy) (*FeeEstimation, error)
+}