User-Provided cdk init Templates

## Description

CDK customers need a way to use and author custom templates for cdk init to meet specific technical/regulatory requirements and standardize project initialization.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @iankhou @rix0rrr
| Author(s) | @rohang9000
| API Bar Raiser | @iliapolo
| Stakeholders | @kaizencc @iliapolo

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

## Workflow

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

Start by reviewing this issue's RFC checklist and the stated customer need for custom templates in cdk init. The payload names no implementation files, tests, or entry points, so identify those from the approved plan before starting. Done should mean the requested template-authoring and initialization capability is implemented and the RFC's completion status is supported by project tests.

Written by the indexing model from the issue text.