cmd/cue/cmd: hook up protobuf encodings types
Closes #606
Change-Id: I712ed6f9fc37d53b633a44ed87e4c71c1e018b0f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9372
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>