chore: generate rust-sdk github action with build.rs

[leoromanovsky]

Observation

• Developed github actions is tedious and slow - validating correctness and behavior has to happen by pushing to remote and waiting for the test to run.
• The yaml syntax needs to be learned.

Desired behavior

• Local validation
• Compiler-guided rule building

Changes proposed

Use the gh_workflow crate in build.rs to automatically keep the github workflow files updated.

Rust's buildscripts (https://doc.rust-lang.org/cargo/reference/build-scripts.html) run on compilation; this PR generates no new work for developers.

Testing

• the ci.yml file should be functionally equivalent
• CI runs - https://github.com/Eppo-exp/eppo-multiplatform/actions/runs/11791022187/job/32842408815?pr=67

[leoromanovsky]

❌ A dependency in this package has a MSRV of 1.75 which blocks us from using it: JelteF/derive_more#389

I will fiddle around with it to see if we can overcome the blocker.

[rasendubi]

I'm actually not a fan of this approach.

It might be cool but it doesn't seem to solve any real issue:

• It doesn't help you to get away from learning yaml and github actions. It just adds a layer of indirection.
  • This is especially true as we have multiple workflow and only one is auto-generated — you still need to know and write yamls. And we have people changing wokrflows who don't have rust knowledge: Quality: Parameterize SDK and test-data branches in CI workflow #73 is changing exactly this file.
• It doesn't help with debugging workflow besides syntax. There doesn't seem to be a command to run it locally — you still need to push your changes to a branch and see if CI passes

and adds a few new:

• Idiomatically, build.rs is designed to configure the compiler and generate source files (which are not commited). It is not designed for generating arbitrary files. One outcome of that is that build.rs runs for users of our library when they install it.
• As yml file is now generated, this adds a bunch of downsides like possible sync issues, people accidentally modifying generated files, slowing down editing (as you have to run it through an additional tool), etc.
• Github actions yaml is more widely known and this is a new tool that everybody has to learn