policy: Add Merge API (#11793)

This commit adds a new API in pkg/bucket/policy package called
Merge to merge multiple policies of a user or a group into one
policy document.

pkg/bucket/policy/actionset.go

@@ -93,10 +93,14 @@ func (actionSet ActionSet) ToSlice() []Action {
 	for action := range actionSet {
 		actions = append(actions, action)
 	}
-
 	return actions
 }
 
+// Clone clones ActionSet structure
+func (actionSet ActionSet) Clone() ActionSet {
+	return NewActionSet(actionSet.ToSlice()...)
+}
+
 // UnmarshalJSON - decodes JSON data to ActionSet.
 func (actionSet *ActionSet) UnmarshalJSON(data []byte) error {
 	var sset set.StringSet