[datadog_application_key] Add preview Actions API access feature

[giorgosdi]

Summary

Adds preview support for Actions API access to both datadog_application_key and datadog_service_account_application_key resources. This feature enables automatic registration and unregistration of application keys for use with Action Connection, App Builder, and Workflow Automation.

Changes

• Added enable_actions_api_access boolean field to both application key resources
• Implemented automatic registration/unregistration with Actions API based on field value
• Added comprehensive acceptance tests for both resources
• Updated documentation and examples
• Defaults to false for backward compatibility
• Proper preview feature labeling

Testing

• ✅ Build passes (make build)
• ✅ Code formatting passes (make fmtcheck)
• ✅ Documentation generated (make docs)
• ✅ Acceptance tests implemented (requires live API for full testing)

Motivation

Addresses issue #3242
Currently non scoped application keys cannot create workflow automations. We need to enable the Actions API per key to be able to do that which can be toggled from the UI but there is no terraform config for it.

[fpighi]

Thanks for the PR. Looks ok, but some tests are failing. Once fixed we can merge.