refactor(object-store): Refactor object store to fit into node framework

[slowli]

What ❔

Refactors ObjectStore and ObjectStoreFactory to fit it into the node framework:

• Consistently uses Arc<dyn ObjectStore> in DI
• Clearly documents ObjectStoreFactory as one possible ObjectStore producer
• Expose GCS, file-based and mock object stores directly and remove ability to create mock object stores from ObjectStoreFactory
• Refactors retries as ObjectStore "middleware"

Why ❔

Currently, object store APIs don't fit into the node framework well, leading to suboptimal DevEx.

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zk fmt and zk lint.
• Spellcheck has been run via zk spellcheck.