(model): standalone py/rust packages for capnproto generated code

[ss2165]

Separate workflows should generate code from capnproto, so that it is not needed as a system dependency for hugr use.
It may be convenient to make the generated code standalone-separately versioned packages.

See: https://github.com/cqcl-dev/quantinuumapis/

[zrho]

As long as the workflow stays optional in the local dev process. Having the capnproto cli tool around at devtime is quite easy to manage; integrating CI workflows when trying out new changes is a lot more tricky.

(Note that the capnproto toolchain isn't a runtime dependency, so it only has to be around when building the packages and not when using them)

[ss2165]

Yeah this is much more worthwhile once the schema itself stops evolving quickly. Once that happens I'm leaning towards moving it in to separate packages, then those packages can be evolved locally as required. CI would just build and publish.