OpenSearch: customEndpoint's CNAME should point to Dual-Stack endpoint when OpenSearch is using Dualstack

### Describe the feature

When an OpenSearch domain is created with a customEndpoint, a CNAME is created pointing to the 'Domain endpoint (IPv4)' (https://[DOMAIN_ID].[REGION].es.amazonaws.com). If the OpenSearchDomain supports DualStack, then the customEndpoint should point to 'Domain endpoint v2 (dual stack)' (https://[DOMAIN_ID].aos.[REGION].on.aws).

### Use Case

We are trying to get rid of IPv4 addresses. During the transition, it would be great to have everything running in dual-stack mode, and later in pure IPv6. Unfortunately, OpenSearch's custom endpoint doesn't currently support dual stack.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.144.0

### Environment details (OS name and version, etc.)

macOS 14.5 23F79 / openjdk version "21.0.3" 2024-04-16

Locate the OpenSearch domain implementation for customEndpoint and its related tests, then inspect how the domain's dual-stack setting and endpoint are represented. Done means a custom endpoint CNAME uses the dual-stack endpoint when dual-stack is enabled, while existing behavior remains correct otherwise, with tests covering the selection.

Written by the indexing model from the issue text.