ENG-872 RDS backup implementation

[terado]

Description

ENG-872 Implement RDS with current blueprint
Implementation of the backup plan inside the source module of the Terraform backup

• variables.tf: RDS config variable (backup_plan_config_rds) with defaults.
• backup_plan.tf: RDS backup plan and selection resources.
• backup_framework.tf: RDS backup framework resource.
• backup_restore_testing.tf: RDS restore testing resource.
• locals.tf: RDS selection tag and framework ARN list entries.
• aws-backups.tf: Default disabled config for RDS in the example module block.
• README.md: RDS resources and input variable documented.

Context

To add RDS backup capability

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.