multi: add label to WalletController SendOutputs and dependent rpcs

Add a label parameter to the WalletController SendOutputs endpoint and
update rpcs that use it to allow optional provision of labels.

labels/labels.go

@@ -3,6 +3,31 @@
 // package to avoid dependency issues.
 package labels
 
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcwallet/wtxmgr"
+)
+
 // External labels a transaction as user initiated via the api. This
 // label is only used when a custom user provided label is not given.
 const External = "external"
+
+// ValidateAPI returns the generic api label if the label provided is empty.
+// This allows us to label all transactions published by the api, even if
+// no label is provided. If a label is provided, it is validated against
+// the known restrictions.
+func ValidateAPI(label string) (string, error) {
+	if len(label) > wtxmgr.TxLabelLimit {
+		return "", fmt.Errorf("label length: %v exceeds "+
+			"limit of %v", len(label), wtxmgr.TxLabelLimit)
+	}
+
+	// If no label was provided by the user, add the generic user
+	// send label.
+	if len(label) == 0 {
+		return External, nil
+	}
+
+	return label, nil
+}