sweep: add lastFeeRate field to PendingInput

This will serve useful when exposing the pending inputs over RPC, since
we currently don't keep track of the last fee rate used for an input.

sweep/sweeper.go

@@ -80,6 +80,10 @@ type pendingInput struct {
 	// map it into a fee rate whenever we attempt to cluster inputs for a
 	// sweep.
 	feePreference FeePreference
+
+	// lastFeeRate is the most recent fee rate used for this input within a
+	// transaction broadcast to the network.
+	lastFeeRate lnwallet.SatPerKWeight
 }
 
 // pendingInputs is a type alias for a set of pending inputs.
@@ -580,6 +584,8 @@ func (s *UtxoSweeper) clusterBySweepFeeRate() []inputCluster {
 			inputs = make(pendingInputs)
 			bucketInputs[bucket] = inputs
 		}
+
+		input.lastFeeRate = feeRate
 		inputs[op] = input
 		inputFeeRates[op] = feeRate
 	}