Rust codegen: expose `bytes::Bytes` for `BLOB` columns

Please add an option to generate bytes::Bytes (instead of Vec<u8>) for binary/blob columns in the Rust bindings produced by spacetime generate.

Why:

1. Vec<u8> forces an allocation and copy whenever row data is cloned. This adds an extra copy to my workflow, which uses async tasks which need to own their data.
2. bytes::Bytes gives zero‑copy, ref‑counted ownership and cheap Clone, reducing latency and allocator pressure for large payloads.
3. Matches the conventions of the wider async‑Rust ecosystem (hyper, tonic, prost, etc.).

Thanks!

Start at the Rust binding code generation path invoked by spacetime generate and trace how BLOB columns are currently mapped to Vec<u8>. Define where the option belongs and verify that generated bindings can use bytes::Bytes; done means the option produces the requested type for binary columns without changing the default behavior.