feat(backup): add GKE backup configuration in the module

[Ameausoone]

Description

This pull request introduces new configuration variables for managing GKE backups:

• backup_cron_schedule: Define backup scheduling as per a cron expression.
• backup_rpo_target_in_minutes: Configure the Recovery Point Objective (RPO).
• backup_config: Specify which volumes or secrets to back up.
• backup_retain_days: Set the retention period for backups.
• resource google_gke_backup_backup_plan to enable backup

Closes https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/issues/2259

Checklist

• [~] Tests added or updated, as applicable. No tests added, but I tested this PR manually, tests seems to not be designed to check other resources than the GKE cluster.
• [x Documentation updated to reflect new configurations.
• [x] Verified and validated new configurations across GKE deployments.

By the way

Integration tests are designed from the ground to be executed in a organisation (which is totally relevant), but it could be difficult (even for professional) to get project creation rights in an organisation. It would be handy to have at least a procedure to execute the tests, in a regular project.