L2 Constructs for AWS Event Schemas

## Description

Create L2 constructs for AWS Event Schemas. This is [tracked in aws-cdk](https://github.com/aws/aws-cdk/issues/6912) and unblocks [schema discovery for event buses](https://github.com/aws/aws-cdk/issues/26480).

## Roles

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

> 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 by reading the linked aws-cdk issue 6912 and the schema-discovery issue 26480 to understand the requested scope. Then follow the RFC Process checklist in this issue; completion means an API-approved RFC, an approved execution plan, and ultimately the implementation-complete status.

Written by the indexing model from the issue text.