fixes aws-controllers-k8s/community#1847

DbClusterParameterGroup was incorrectly rejecting valid parameters like slow_query_log, long_query_time, and log_queries_not_using_indexes with "unknown parameter" errors. This occurred because the controller only validated parameters against DescribeEngineDefaultClusterParameters API, but some valid cluster parameters (particularly MySQL logging parameters) are only listed in the DescribeEngineDefaultParameters (instance-level) API response.

Description of changes:

Implemented a fallback parameter validation mechanism in getParameterMeta() that first checks cluster-level parameter defaults, and if not found, falls back to instance-level parameter defaults before rejecting a parameter as unknown. This allows the controller to properly validate and apply parameters that AWS RDS Console and CLI support but weren't previously accessible through the ACK controller, while maintaining backward compatibility and proper parameter metadata handling for apply methods and modifiability checks.

apiVersion: rds.services.k8s.aws/v1alpha1
kind: DBClusterParameterGroup
metadata:
  name: test-slow-query-log
spec:
  name: test-slow-query-log
  description: Test parameter group to reproduce slow_query_log issue
  family: "aurora-mysql8.0"
  parameterOverrides:
    slow_query_log: "1"
    long_query_time: "10"
    log_queries_not_using_indexes: "1"

{"level":"debug","ts":"2025-07-21T14:41:49.089-0700","logger":"ackrt","msg":"patched resource status","kind":"DBClusterParameterGroup","namespace":"default","name":"test-slow-query-log","account":"xxx","role":"","region":"us-west-2","is_adopted":false,"generation":1,"json":"{\"metadata\":{\"resourceVersion\":\"103993920\"},\"spec\":{\"tags\":null},\"status\":{\"ackResourceMetadata\":{\"arn\":\"arn:aws:rds:us-west-2:xxxx:cluster-pg:test-slow-query-log\",\"ownerAccountID\":\"xxxx\",\"region\":\"us-west-2\"},\"conditions\":[{\"lastTransitionTime\":\"2025-07-21T21:41:49Z\",\"message\":\"Resource synced successfully\",\"reason\":\"\",\"status\":\"True\",\"type\":\"ACK.ResourceSynced\"}],\"parameterOverrideStatuses\":[{\"applyMethod\":\"immediate\",\"applyType\":\"dynamic\",\"parameterName\":\"log_queries_not_using_indexes\",\"parameterValue\":\"1\"},{\"applyMethod\":\"immediate\",\"applyType\":\"dynamic\",\"parameterName\":\"long_query_time\",\"parameterValue\":\"10\"},{\"applyMethod\":\"immediate\",\"applyType\":\"dynamic\",\"parameterName\":\"slow_query_log\",\"parameterValue\":\"1\"}]}}"}

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