docs: add ADR for filesystem payload offloading

[zhongkechen]

Summary

• add ADR-005 for filesystem payload offloading design
• compare reusing SerDes with introducing a PayloadOffloader interface
• document extra package naming, caching, threading, exception handling, and AI recommendation

Preview

https://github.com/aws/aws-durable-execution-sdk-java/blob/codex/adr-filesystem-payload-offloading/docs/adr/005-filesystem-serdes.md

Issue

• Refs Add FileSystem serdes #463

Testing

• not run; docs-only change

[SilanHe]

I like that we're discussing this decision in open source.

So are you proposing Approach B : Create a PayloadOffloader Interface as is recommended by the AI?