feat(bucket-object): add object metadata [15/n]

[sadhansood]

Description

This adds first-class custom metadata to bucket_object.

The main changes are:

• a new ObjectMetadata module backed by VecMap<String, String>
• ObjectVersion now stores ObjectMetadata
• the register-and-stage write APIs now require metadata at version creation time
• delete markers carry empty metadata by construction

This keeps user metadata part of the immutable object-version snapshot, which gives us a stable base for later metadata-only update APIs, copy semantics, and gateway-facing HeadObject behavior.

Test plan

• rustup run 1.86 cargo fmt -- --config group_imports=StdExternalCrate,imports_granularity=Crate,imports_layout=HorizontalVertical
• cargo clippy --workspace --all-features --tests --no-deps -- -D warnings
• sui move test --path /private/tmp/walrus-bucket-object-15/contracts/bucket_object

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