[Resource Type] - [Coverage] - Support flat-rate pricing plans (PricingPlanManager)

### Name of the resource

Other

### Resource name

_No response_

### Description

CloudFront flat-rate pricing plans (Free / Pro / Business / Premium), launched
in November 2025, can currently only be managed through the CloudFront console.
There is no native way to subscribe, upgrade, downgrade, or cancel a plan via
CloudFormation.

These plans are managed by a separate service, PricingPlanManager
(eventSource: pricingplanmanager.amazonaws.com), NOT through the CloudFront
distribution API. The public DistributionConfig API has no pricing-plan field,
and there is no CloudFormation resource type for PricingPlanManager today. The
service is also not yet available in the AWS SDKs / CLI, so the only workaround
today is a Lambda-backed Custom Resource that calls the pricingplanmanager
endpoint directly (see the related CDK issue) — which is hacky and undesirable
for production IaC.

Requesting a new CloudFormation resource type (e.g. AWS::CloudFront::PricingPlan
or an AWS::PricingPlanManager::* resource) to manage flat-rate pricing plan
subscriptions natively, supporting:
- Plan tier selection (Free / Pro / Business / Premium)
- Premium usage-level configuration
- Association with a distribution / apex domain
- Subscribe / upgrade / downgrade / cancel lifecycle

### Other Details

References:
- Docs: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/flat-rate-pricing-plan.html
- PricingPlanManager User Guide: https://docs.aws.amazon.com/PricingPlanManager/latest/UserGuide/
- Related CDK request: https://github.com/aws/aws-cdk/issues/37857
- cfn-lint already sees new pricingplanmanager events: https://github.com/aws-cloudformation/cfn-lint/issues/4303

Read the flat-rate pricing-plan documentation and the related CDK issue first, then compare the cfn-lint PricingPlanManager event coverage mentioned here. Done means a reviewed native CloudFormation resource design covering tier selection, premium usage, distribution or apex-domain association, and the subscribe, upgrade, downgrade, and cancel lifecycle.

Written by the indexing model from the issue text.