feat: add RDS instance class modification

[nathanhuh]

Problem

unic can inspect and operate on RDS instances, but it cannot resize an instance by modifying its DB instance class.

Scope

• Add a modify-size action from RDS instance detail.
• Load valid DB instance class options for the selected engine/engine version/region where AWS exposes them.
• Present options in a filterable picker using existing list behavior, including wrap navigation and empty states.
• Show enough metadata to choose safely, such as class name and any available engine/storage/license constraints.
• Add a confirmation step before calling ModifyDBInstance.
• Make apply-immediately behavior explicit, ideally as a selectable/toggleable choice before confirmation.
• Refresh or poll the selected RDS instance after submitting the modification.

Acceptance Criteria

• RDS detail exposes a clear resize/modify-class action.
• Users can filter available instance classes and choose one.
• The selected class and apply-immediately choice are shown in a confirmation screen.
• The modify call is protected by the existing type-to-confirm pattern or equivalent explicit confirmation.
• API errors and unsupported/no-option cases render actionable messages.
• Tests cover option loading/mapping, filtering, confirmation validation, and successful/error modify flows.
• README documents the RDS resize flow.
• make test and make build pass.