AWS CloudWatch Application Signals L2 Constructs for SLO

## Description

Add L2 Constructs for AWS CloudWatch Application Signals SLO to simplify the creation of Application Signals SLO resources.
See https://github.com/aws/aws-cdk-rfcs/issues/670 for
additional details.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @liunia-amazon, @bjrara
| Author(s) | @liunia-amazon
| API Bar Raiser | @moelasmar
| Stakeholders | @alias, @alias, @alias

> 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`)
- [ ] 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 RFC 670 and the approved execution plan referenced by the issue; no implementation files, tests, or entry points are named. Done means adding the AWS CloudWatch Application Signals SLO L2 Constructs and completing the implementation checklist.

Written by the indexing model from the issue text.