custom update code path for DBCluster (#24)

The ModifyDBCluster RDS API call will return an InvalidParameterValue
error if the engine mode for the DBCluster is "serverless" and any of
the following fields have been set on the ModifyDBClusterInput shape:

* PreferredMaintenanceWindow
* PreferredBackupWindow
* Port

This error is returned **even if the value of the field in the
ModifyDBClusterInput shape matches the existing value of the DBCluster
resource**

To get around this issue, I've added a custom update method
implementation for the DBCluster resource that builds the
ModifyDBClusterInput shape and ONLY sets fields on that shape when the
matching Spec field is not nil AND there is a difference detected
between the latest observed value and the desired value for that field.

Fixes Issue aws-controllers-k8s/community#917

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

Author: jaypipes

apis/v1alpha1/ack-generate-metadata.yaml

@@ -1,14 +1,14 @@
 ack_generate_info:
-  build_date: "2021-08-24T15:21:38Z"
+  build_date: "2021-08-26T13:53:10Z"
   build_hash: 11f7affb243007aa3ed1140e7093d8e0ab1252fd
   go_version: go1.15.5 linux/amd64
   version: v0.12.0
-api_directory_checksum: 4bda8793ed6390385882d640e1e7bf541a342c0f
+api_directory_checksum: 387f19255122f7e68716af4d6dbce0e7498cfe37
 api_version: v1alpha1
 aws_sdk_go_version: v1.37.10
 generator_config_info:
-  file_checksum: bb80f830d61e1a1c6016fde6102463862f9dc8dd
+  file_checksum: a925bf4c19337d7d6e61ae88861f00a1e94cf7a1
   original_file_name: generator.yaml
 last_modification:
   reason: API generation
-  timestamp: 2021-08-24 15:21:45.484880224 +0000 UTC
+  timestamp: 2021-08-26 13:53:17.093835611 +0000 UTC

apis/v1alpha1/generator.yaml

@@ -64,6 +64,11 @@ operations:
     primary_identifier_field_name: DBClusterIdentifier
 resources:
   DBCluster:
+    update_operation:
+      # We need a custom update implementation until the issue behind
+      # https://github.com/aws-controllers-k8s/community/issues/917 is
+      # resolved.
+      custom_method_name: customUpdate
     hooks:
       sdk_update_pre_build_request:
         template_path: hooks/db_cluster/sdk_update_pre_build_request.go.tpl

generator.yaml

@@ -64,6 +64,11 @@ operations:
     primary_identifier_field_name: DBClusterIdentifier
 resources:
   DBCluster:
+    update_operation:
+      # We need a custom update implementation until the issue behind
+      # https://github.com/aws-controllers-k8s/community/issues/917 is
+      # resolved.
+      custom_method_name: customUpdate
     hooks:
       sdk_update_pre_build_request:
         template_path: hooks/db_cluster/sdk_update_pre_build_request.go.tpl