View types lack serde::Serialize, forcing to_owned() for JSON

[iainmcgin]

Generated *View<'a> types implement ViewEncode (proto-binary) but not serde::Serialize, so a consumer that wants to emit JSON from a borrowed view has to to_owned() first.

In connect-rust this surfaces as an asymmetry in the handler API: a handler can return a view body for proto clients (zero-copy via ViewEncode), but the same handler returns Code::Unimplemented to a JSON client because there's no way to serialize the view. The owned type round-trips fine in both codecs.

// connectrpc::__codegen::encode_view_body, simplified:
match codec {
    CodecFormat::Proto => Ok(v.encode_to_bytes()),
    CodecFormat::Json  => Err(ConnectError::unimplemented(
        "view-body responses do not support the JSON codec; \
         return the owned message type for JSON-serving handlers",
    )),
}

Emitting impl Serialize for FooView<'_> (gated behind the existing json feature, like the owned types) would close the gap. The view already has all the field data; the impl is structurally the same as the owned one minus the Option/presence wrapping differences.

This is larger than the other two asks (touches every view emitter) so flagging it as a feature request rather than a 0.4.x candidate, unless adjacent serde work is already planned.