apigateway: private domain name L2 construct

## Description

As initially proposed here aws/aws-cdk/issues/33352, the goal is to create a set of L2 constructs for [API Gateway custom domain names for private APIS](https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-private-custom-domains.html). The three proposed constructs would be as follows:

- `PrivateDomainName` maps to `CfnDomainNameV2`
- `PrivateBasePathMapping` maps to `CfnBasePathMappingV2`
- `DomainNameAccessAssociation` maps to `CfnDomainNameAccessAssociation`

The resulting solution would provide a straightforward path for users to launch private apis with custom domain names that matches the developer experience already available for public custom domain names.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @jtmthf
| Author(s) | @jtmthf
| API Bar Raiser |
| Stakeholders |

> See [RFC Process](https://github.com/aws/aws-cdk-rfcs#rfc-process) for details

## Workflow

- [x] Tracking issue created (label: `status/proposed`)
- [ ] API bar raiser assigned (ping us at [#aws-cdk-rfcs](https://cdk-dev.slack.com/archives/C025ZFGMUCD) if needed)
- [ ] Kick off meeting
- [ ] RFC pull request submitted (label: `status/review`)
- [ ] Community reach out (via Slack and/or Twitter)
- [ ] API signed-off (label `status/api-approved` applied to pull request)
- [ ] Final comments period (label: `status/final-comments-period`)
- [ ] Approved and merged (label: `status/approved`)
- [ ] Execution plan submitted (label: `status/planning`)
- [ ] Plan approved and merged (label: `status/implementing`)
- [ ] Implementation complete (label: `status/done`)

---

> Author is responsible to progress the RFC according to this checklist, and
apply the relevant labels to this issue so that the RFC table in README gets
updated.

Start with the proposal in aws/aws-cdk/issues/33352 and the linked AWS documentation for private custom domains. The issue names the proposed constructs and their CloudFormation mappings, but no repository files or tests; first clarify the API design and advance the RFC checklist. Done means the proposal reaches an approved execution plan or implementation completion.

Written by the indexing model from the issue text.