autopilot: add ExternalScoreAttachment heuristic

This commit adds a new autopilot heuristic that is scoring based. It is
a simple heuristic that will keep a list of pubkeys and scores, and will
try opening channels with the nodes with the largest score first.

autopilot/interface.go

@@ -147,6 +147,7 @@ var (
 	// with the autopilot agent.
 	availableHeuristics = []AttachmentHeuristic{
 		NewPrefAttachment(),
+		NewExternalScoreAttachment(),
 	}
 
 	// AvailableHeuristics is a map that holds the name of available