Add support for Amazon API Gateway REST API

## Description
Request to add support for Amazon API Gateway REST APIs to the CloudWatch Dashboard Generator tool.
API Gateway is a commonly used service for building serverless APIs that would benefit from monitoring during Countdown scenarios.

## Proposed Implementation
Add API Gateway REST API support with the following dimensions:

- **Namespace**: `AWS/ApiGateway`
- **Dimensions**:
- `ApiName` and `Stage`
- **Metrics**:
- Count (number of API calls)
- Latency (overall latency of API requests)
- IntegrationLatency (latency of backend integration)
- 4XXError (client-side errors)
- 5XXError (server-side errors)

## Expected CSV Format
service,region_code,dim1_name,dim1_value,dim2_name,dim2_value
APIGW_REST,us-east-1,ApiName,my-rest-api,Stage,prod
APIGW_REST,us-east-1,ApiName,my-rest-api,Stage,dev

Start by locating the existing service implementations and the CLI's CSV input and dashboard-generation entry points. Add the AWS/ApiGateway service using the ApiName and Stage dimensions and listed metrics, then verify that the documented APIGW_REST CSV rows produce the expected dashboard configuration.

Written by the indexing model from the issue text.