L2 construct for CloudWatch Promql alarms

## Description

A new PromQLAlarm L2 construct in aws-cdk-lib/aws-cloudwatch that lets users create CloudWatch alarms based on PromQL queries. It extends AlarmBase (alongside Alarm and CompositeAlarm), supports query, evaluationInterval, pendingPeriod, and recoveryPeriod properties, and maps to `AWS::CloudWatch::Alarm` using the `EvaluationCriteria.PromQLCriteria` configuration.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @caoxy
| Author(s) | @caoxy
| API Bar Raiser | tbd
| Stakeholders | @tiagocb, @byrneter

> 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 reviewing the proposed PromQLAlarm L2 construct in aws-cdk-lib/aws-cloudwatch and comparing the mentioned AlarmBase, Alarm, and CompositeAlarm APIs. Check how the construct should map to AWS::CloudWatch::Alarm through EvaluationCriteria.PromQLCriteria; done requires an approved API and implementation plan covering query, evaluationInterval, pendingPeriod, and recoveryPeriod.

Written by the indexing model from the issue text.