loopdb: add helper methods to update swap costs

This commit adds the necessary sqlc code and SwapStore function to
update swap costs for all swaps in one transaction.

loopdb/interface.go

@@ -65,6 +65,11 @@ type SwapStore interface {
 	// it's decoding using the proto package's `Unmarshal` method.
 	FetchLiquidityParams(ctx context.Context) ([]byte, error)
 
+	// BatchUpdateLoopOutSwapCosts updates the swap costs for a batch of
+	// loop out swaps.
+	BatchUpdateLoopOutSwapCosts(ctx context.Context,
+		swaps map[lntypes.Hash]SwapCost) error
+
 	// Close closes the underlying database.
 	Close() error
 }