feat: Add RDS ListDBInstances support

[YoungJinJung]

Summary

Add support for listing RDS DB instances in the TUI.

Details

• Add RDS service and ListDBInstances feature to the catalog (internal/domain/)
• Implement ListDBInstances in internal/services/aws/rds.go using aws-sdk-go-v2/service/rds
• Add screen transition for the RDS feature in internal/app/actions.go
• Display DB instance details (identifier, engine, status, endpoint, instance class, etc.)

Checklist

• Add AwsService / FeatureKind constants in internal/domain/model.go
• Register in catalog via internal/domain/catalog.go
• Implement AwsRepository.ListDBInstances() in internal/services/aws/rds.go
• Add screen transition in internal/app/actions.go
• Add new screen model if needed in internal/app/screens.go
• Add rds SDK dependency in go.mod
• Write tests

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.

[nathanhuh]

Needed to reopen after found a fundamental scope issue with not supporting Aurora Cluster for RDS related operations.