helper: sanitize method on ACL token object

There are several places where we want to redact the secret ID of an ACL token,
some of which are in the Enterprise code base for Sentinel. Add a new method
`Sanitize` that mirrors the one we have on `Node`.

Ref: hashicorp/nomad-enterprise#2087

nomad/structs/acl.go

@@ -474,6 +474,16 @@ func (a *ACLToken) UnmarshalJSON(data []byte) (err error) {
 	return nil
 }
 
+func (a *ACLToken) Sanitize() *ACLToken {
+	if a == nil {
+		return nil
+	}
+
+	out := a.Copy()
+	out.SecretID = ""
+	return out
+}
+
 // ACLRole is an abstraction for the ACL system which allows the grouping of
 // ACL policies into a single object. ACL tokens can be created and linked to
 // a role; the token then inherits all the permissions granted by the policies.

nomad/structs/event.go

@@ -155,8 +155,7 @@ type ServiceRegistrationStreamEvent struct {
 // NewACLTokenEvent takes a token and creates a new ACLTokenEvent.  It creates
 // a copy of the passed in ACLToken and empties out the copied tokens SecretID
 func NewACLTokenEvent(token *ACLToken) *ACLTokenEvent {
-	c := token.Copy()
-	c.SecretID = ""
+	c := token.Sanitize()
 
 	return &ACLTokenEvent{
 		ACLToken: c,