feat: add RDS cluster parameter group actions

[john-goloti]

Summary

Implements the five missing Aurora cluster parameter group APIs in the RDS Query handler:

• CreateDBClusterParameterGroup
• DescribeDBClusterParameterGroups
• DeleteDBClusterParameterGroup
• ModifyDBClusterParameterGroup
• DescribeDBClusterParameters

Motivation

Stacks defined with the Terraform/Pulumi AWS provider that declare an aws_rds_cluster_parameter_group (or Pulumi's aws.rds.ClusterParameterGroup) currently fail against Floci with:

api error UnsupportedOperation: Operation CreateDBClusterParameterGroup is not supported.

This prevents Aurora cluster deployment from completing, since the cluster resource references the parameter group via db_cluster_parameter_group_name. Floci already implements the instance-level parameter group APIs (CreateDBParameterGroup and friends); this PR adds the cluster-level counterparts that the AWS service model exposes as a parallel resource type.

Implementation

• New model class DbClusterParameterGroup (mirrors DbParameterGroup).
• RdsService gains an in-memory StorageBackend<String, DbClusterParameterGroup> and CRUD methods.
• RdsQueryHandler dispatches the five new actions, writes the appropriate XML envelopes (<DBClusterParameterGroup> / <DBClusterParameterGroups> / <Parameters>), and matches the existing parameter-group code style.
• No changes to existing instance-level behavior.

Tests

• RdsServiceTest: round-trip, duplicate rejection, modify parameters, missing-name errors for delete/get.
• RdsQueryHandlerTest: tag-shape assertions, required-field validation for create/delete/describe, argument passthrough, parameter parsing that ignores entries with no value.

./mvnw test -Dtest='RdsServiceTest,RdsQueryHandlerTest' runs 31 tests, 0 failures.

Verified against a real IaC deploy

The original blocker came from a Pulumi infrastructure-application stack that declared:

new aws.rds.ClusterParameterGroup("aurora-parameter-group-17", {
  family: "aurora-postgresql17",
  parameters: [
    { name: "log_statement", value: "all" },
    { name: "log_min_duration_statement", value: "500" },
    { name: "shared_preload_libraries", value: "pg_stat_statements" },
    { name: "rds.force_ssl", value: "0" },
  ],
});

With Floci 1.5.15 this errored before any Aurora resource was created. With this PR's branch built locally, the action succeeds and ModifyDBClusterParameterGroup accepts the four parameters above.

Out of scope

• Cluster parameter group family validation — accepted as-is, matching the leniency of the instance-level implementation.
• Tags on parameter groups — neither the instance-level handler nor this one currently emits them in the response (kept consistent for now; can be a follow-up).
• Source attribute on individual parameters (engine-default / user) — emitted only as IsModifiable=true here, same as DescribeDBParameters.

[hectorvent]

Thanks @john-goloti

Solid implementation. Great job!