Fix No modifications were requested issue during sdkUpdate for CacheSubnetGroup (#112)

**Description of changes:**

- Encountered `InvalidParameterValue: No modifications were requested.` during `sdkUpdate` just after an adoption of an existing resource. I introduced a custom update for the `CacheSubnetGroup` resource in order to swallow the error during `sdkUpdate`. Waiting your comments/reviews in case there is a better approach to handle this issue.

**For more context:** #111 (review)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Author: mustafasencer

apis/v1alpha1/ack-generate-metadata.yaml

@@ -1,5 +1,5 @@
 ack_generate_info:
-  build_date: "2023-03-10T17:52:20Z"
+  build_date: "2023-03-13T12:38:41Z"
   build_hash: 910a8e0744a99c5c87d8c1615926985215a60d8c
   go_version: go1.19
   version: v0.24.3

pkg/resource/cache_subnet_group/sdk.go

@@ -1,6 +1,8 @@
 
-        for _, subnetIdIter := range ko.Status.Subnets {
-                if subnetIdIter.SubnetIdentifier != nil {
-                        ko.Spec.SubnetIDs = append(ko.Spec.SubnetIDs, subnetIdIter.SubnetIdentifier)
-                }
-        }
+	subnets := make([]*string, 0, len(ko.Status.Subnets))
+	for _, subnetIdIter := range ko.Status.Subnets {
+		if subnetIdIter.SubnetIdentifier != nil {
+			subnets = append(subnets, subnetIdIter.SubnetIdentifier)
+		}
+	}
+	ko.Spec.SubnetIDs = subnets