✨ feat: add capacity reservation group support

[justinmir]

What type of PR is this?

/kind feature
/kind api-change

What this PR does / why we need it:

Adds support for targeting AWS Capacity Reservation resource groups from CAPA launch paths.

This adds a v1beta2 capacityReservationResourceGroupARN field for AWSMachine and AWSLaunchTemplate, then plumbs it through EC2 instance and launch template request generation. The new target is mutually exclusive with capacityReservationId and follows the existing Spot and CapacityBlock validation rules.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
N/A

Special notes for your reviewer:

This mirrors the existing capacity reservation ID behavior, but sets CapacityReservationResourceGroupArn on the AWS SDK capacity reservation target.

AI Usage:

This PR was developed with assistance from GPT-5.5 with the Opencode Harness.

Checklist:

• squashed commits
• includes documentation
• includes AI generated content
• includes emoji in title
• adds unit tests
• adds or updates e2e tests

Release note:

Added support for targeting AWS Capacity Reservation resource groups from AWSMachine and AWSMachinePool launch templates.

[k8s-ci-robot]

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