Investigate pre-generating Rust protobuf code

[hug-dev]

Similarly as the discussion about this topic on the Go client PR, we should investigate if it's good and worth it to pre-generate the code from our protobuf files and check it in tree instead of generating it for everybuild.

Since our contracts are stable and backward-compatible by design, the main advantage of this is winning compilation time by requiring less build step and less dependencies. Seeing the number of times those files are generated, I am sure we could see a huge advantage.

The protobuf code could be generated under a feature disabled by default (by default use the pre-generated one) and the CI could check that the generated code is the one we expect.

[hug-dev]

Putting it as "good-first-issue" as I think we can just go with it and pre-generate the contracts.