Fields of `bytes` should be percent-encoded as query parameters

To send query parameters of type "object" we serialize the object as JSON and send expand the parameters:

https://github.com/googleapis/google-cloud-rust/blob/0b3d43254520095682b9717643d0b37de218a371/generator/internal/rust/codec.go#L694-L702

The resulting code looks like so:

https://github.com/googleapis/google-cloud-rust/blob/0b3d43254520095682b9717643d0b37de218a371/src/generated/showcase/src/transport.rs#L115-L124

Consequently fields of type `bytes` are base64-encoded. Looking at how [showcase](https://github.com/googleapis/gapic-showcase/blob/60f4159fbf6474304c8b0c088c28e77647763e83/util/genrest/resttools/populatefield.go#L166-L167) works, it seems that these should be percent-encoded (which makes sense for query parameters).