(AWS Elemental MediaPackage V2 + Amazon CloudFront Origins): Extending stable module functionality into alpha modules

## Description

RFC to cover the idea/design of extending stable patterns into alpha modules (as we can't do vice versa).
Covering AWS Elemental MediaPackage V2 (alpha) and Amazon CloudFront (stable) modules.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @jamiepmullan
| Author(s) | @jamiepmullan
| API Bar Raiser | @alvazjor
| 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`)
- [x] 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.

Begin with the RFC Process linked in the issue and review the workflow checklist. No file, test, entry point, or RFC pull request is named; the proposed scope concerns extending stable patterns from Amazon CloudFront into the alpha AWS Elemental MediaPackage V2 module, with completion represented by the listed RFC status progression.

Written by the indexing model from the issue text.