Add audit history and events for Cloud Manager environment variable changes

Request for Feature Enhancement (RFE) Summary:
Provide an audit trail for environment variable changes in Cloud Manager, accessible through the Cloud Manager UI.

Use case:
As an AEM as a Cloud Service administrator, I need to identify who created, updated, or deleted an environment variable and when. These changes can affect application behavior without a code deployment, making traceability essential for troubleshooting, incident response, and change management.

Current/Experienced Behavior:
The documented environment variables API exposes the current configuration but does not provide the user responsible for a change, its timestamp, or revision history. The documented Cloud Manager events focus on pipeline activity and do not include environment variable changes.

Polling the API cannot reliably identify the actor or exact change time, can miss intermediate changes, and cannot detect secret-value changes reliably.

Improved/Expected Behavior:

• Record variable creation, updates, and deletion, whether performed through the Cloud Manager UI, API, or CLI.
• Include the actor’s identity, UTC timestamp, program/environment IDs, variable name, service scope, operation, and outcome.
• Distinguish human users from technical accounts.
• Provide a searchable audit history in Cloud Manager.
• Expose audit records through a paginated API with filtering by date, environment, variable, and actor.
• Publish corresponding Adobe I/O Events for integration with monitoring and audit platforms.
• Document retention periods and support exporting records for longer-term storage.

For secret variables, record that a change occurred and its metadata without exposing previous or new secret values. For regular variables, provide before/after values subject to appropriate access controls.

Business impact:
This would help teams investigate configuration-related incidents, identify unauthorized changes, and maintain accountability without requiring a custom tool to mediate every configuration update.

Environment:
Adobe Experience Manager as a Cloud Service — Cloud Manager.

Example:
If an external API credential is updated and an integration subsequently fails, administrators should be able to establish who updated the variable, when it happened, which environment and service were affected, and whether the update completed successfully—without revealing the credential.