perf: resmap: Add AppendMany function

Introduce a new function, AppendMany, which can be used to safely add
entries to a resmap.

The function handles ID conflict detection, which was the most expensive
operation causing significant performance issues.

The ID cache is local to this function as we cannot guarantee that all
mutations are performed using this function.

api/resmap/resmap.go

@@ -133,6 +133,8 @@ type ResMap interface {
 	// in the cluster with the same Id.
 	Append(*resource.Resource) error
 
+	AppendMany(res ...*resource.Resource) error
+
 	// AppendAll appends another ResMap to self,
 	// failing on any CurId collision.
 	AppendAll(ResMap) error

api/resmap/reswrangler.go

@@ -83,6 +83,27 @@ func (m *resWrangler) Append(res *resource.Resource) error {
 	return nil
 }
 
+// Append implements ResMap.
+func (m *resWrangler) AppendMany(resources ...*resource.Resource) error {
+	ids := m.AllIds()
+	seen := make(map[resid.ResId]struct{}, len(ids)+len(resources))
+	for _, id := range ids {
+		seen[id] = struct{}{}
+	}
+
+	for _, res := range resources {
+		newId := res.CurId()
+		if _, ok := seen[newId]; ok {
+			return fmt.Errorf(
+				"may not add resource with an already registered id: %s", newId)
+		}
+		seen[newId] = struct{}{}
+		m.append(res)
+	}
+
+	return nil
+}
+
 // append appends without performing an Id check
 func (m *resWrangler) append(res *resource.Resource) {
 	m.rList = append(m.rList, res)