fix: add body_json and content_type to queue messages

[SameerVers3]

Closes #1013

serde_wasm_bindgen::from_value is unreliable for types like serde_json::Value and tuples. This adds an escape hatch:

for msg in batch.raw_iter() {
    let body: MyPayload = msg.body_json()?;
}

Also exposes content_type() on Message and RawMessage so consumers can inspect the producer's format.