invoicesrpc: add SubscribeSingleInvoice rpc

lnrpc/file_utils.go

@@ -0,0 +1,15 @@
+package lnrpc
+
+import (
+	"os"
+)
+
+// FileExists reports whether the named file or directory exists.
+func FileExists(name string) bool {
+	if _, err := os.Stat(name); err != nil {
+		if os.IsNotExist(err) {
+			return false
+		}
+	}
+	return true
+}