AWS CloudWatch Application Signals L2 Constructs

## Description

Add L2 Constructs for AWS CloudWatch Application Signals to simplify the enablement steps and the creation of Application Signals resources.

## Roles

| Role | User
|---------------------|------------------------------
| Proposed by | @bjrara, @liunia-amazon
| Author(s) | @bjrara, @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 by reviewing the approved RFC and its execution plan, then trace the Application Signals enablement steps and resource-creation requirements described there. Done means the L2 Constructs implementation is complete and the issue can be marked `status/done`.

Written by the indexing model from the issue text.