feat(storage): include custom object metadata in ObjectHighlights

[metebalci]

Is your feature request related to a problem? Please describe.

ObjectHighlights returned by Storage::read_object(...).send().await?.object() omits the metadata: HashMap<String, String> field of Object.
Reading per-object custom metadata requires a follow-up StorageControl::get_object RPC — doubling the GET count on workloads that key on per-object metadata, even though the metadata is already on the wire
alongside the body:

• gRPC: google.storage.v2.ReadObjectResponse field 4 is metadata: Object, sent on the first response message of the stream.
• REST: GET /storage/v1/b/{bucket}/o/{object}?alt=media surfaces custom metadata as x-goog-meta-* HTTP response headers.

Describe the solution you'd like

Add metadata: HashMap<String, String> to ObjectHighlights. Additive on a #[non_exhaustive] struct, no API break. Optionally also surface event_based_hold, temporary_hold, retention so the response can drive retention/hold gating without a follow-up call.

Describe alternatives you've considered

Issuing the follow-up StorageControl::get_object per download — the current workaround the doc comment recommends. Works, but doubles the request count.

Additional context

ObjectHighlights was introduced in #2626 / #2685 as a deliberately curated subset, so this is asking to expand the subset, not redesign the API. The two other major-cloud Rust SDKs already expose object metadata on their download responses:

• AWS — GetObjectOutput::metadata: Option<HashMap<String, String>>
• Azure — BlobClient::download response properties surface x-ms-meta-*