feat: implement retention command

[overtrue]

Description

Implement the retention command for managing S3 Object Lock retention policies.

Requirements

• Support Object Lock retention modes (Governance/Compliance)
• Ability to set/get/remove retention on objects
• Requires S3 Object Lock feature support from the storage provider

Acceptance Criteria

• rc retention set - Set retention policy on object
• rc retention get - Get retention policy of object
• rc retention remove - Remove retention policy (Governance mode only)
• Return EXIT_UNSUPPORTED_FEATURE (7) when Object Lock is not supported
• Support --force flag to bypass capability detection

Notes

This is a capability-dependent feature. Not all S3-compatible storage providers support Object Lock.