fix(rds): circular dependencies when creating multiple DatabaseProxies (#28471)

### Description
The related issue reports that deployment fails due to circular dependencies when multiple RDSProxy are created.
The `DatabaseProxy` uses the `node.addDependency` method to ensure that the `CfnDBProxyTargetGroup` is created after the `DBCluster` and `DBInstance` are created (#12237).

This works well for a single `DatabaseProxy`, but does not work well when multiple `DatabaseProxy` are created with `DatabaseCluster.addProxy`.
When creating a `DatabaseProxy` with the `DatabaseCluster.addProxy` method, it is created as a child of the `DatabaseCluster`.
https://github.com/aws/aws-cdk/blob/cd54c4239ec29182e30fd91634505df560d6e5f8/packages/aws-cdk-lib/aws-rds/lib/cluster.ts#L446

The `node.addDependency` method recursively sets dependencies on child Constructs, so if multiple `DatabaseProxy` are created as a child of a `DatabaseCluster` in the construct tree, multiple `DatabaseProxy` dependencies on each other.
If the `addProxy` method is not used, the user initializes the `DatabaseProxy` directly and it does not become a child of `DatabaseCluster`.
For example,
```ts
new DatabaseProxy(stack, 'DBProxy', {
  proxyTarget: rds.ProxyTarget.fromCluster(cluster),
  vpc,
});
```

I believe this is the cause of the `these resources have a dependency cycle` error reported in the related issue.

To correct this error, this PR uses `CfnResource.addDependency` instead of `node.addDependency` to avoid recurrent dependencies.

Closes #25633

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

Author: sakurai-ryo