Add SwappableObjectStore for runtime client reconfiguration

[andrewn6]

Summary

• Adding SwappableObjectStore, an ObjectStore wrapper, supports atomically swapping the inner store at runtime
• Enabled future work to pick up S3 Credentials/endpoint & changes from config file without a process restart

Design

• SwappableObjectStore wraps Arc<dyn ObjectStore> inside an ArcSwap and delegates all 10 ObjectStore trait methods. Each method calls load_full() to get an owned Arc that is safe to hold across .await points and in returned streams.

This tackles issue: #3615

ran fo

A few notes:

• This is just a foundational piece on how we could potential approach this, once agreed on we'd need to wire into consumers and adding a config watcher (not sure if there's a sub-issue of this), probably could be a seperate PR..
• No unit test, happy to add one using InMemory stores if wanted!

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[andrewn6]

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