fix: move proto code generation output to OUT_DIR, out of tree

[drahnr]

Motivation

It's considered bad practice to commit bad practice to commit generated files into the source tree, the decision predates me.
We're effectively dropping 10k from our codebase and avoid a little friction on changes in protobuf.
Any decent editor that's using rust-analyzer can resolve include! directives, so the location isn't relevant anymore.

Note

We have a bit of machinery built around the proto generation, so I kept the integration bits by using the generated/mod.rs to pull in concat!(env!(OUT_DIR), "/generated/..*") files. The main purpose is to minimize the already gigantic changeset and avoid having to adapt import paths across the repository.

Open Questions

While this is desirable in principle, it implies

Side effects

We are effectively dropping the special casing of docs.rs builds based on BUILD_PROTO=1