Move build logic

[Fantoni0]

As @v0-e suggested:

moving circuits/build.rs logic into its own binary. Circuit artifacts are such a central part of our process and safety/security, and a dedicated executable could be a more idiomatic design.

Right now circuits/build.rs is a lib crate with a build.rs script. If it were made a binary crate with a wrapper bash script, we could trigger this behavior in other parts of the codebase, and not only when the circuits crate is compiled.
An example could be publishing compiled artifacts in CI rather than having them be part of the committed codebase.

Not urgent. But it would be nice to have.

[RajeshRk18]

I can take this up! @Fantoni0 @v0-e

• First, I would add a cli binary crate which helps us with fetching and pinning circomlib and other artifact build steps(compile, generating keys, graphs, and verify).
• Emit manifest.json into artifacts directory. For every circuit built, it will have sha256 and length of the .r1cs and .wasm, circom compiler tag, circomlib.lock revision, prime, and hashes of the source .circom files. With this, we can make sdk/web/build.rs read the hashes and stem lists from manifest.json.
• I would add a wrapper script in scripts/ which will compile the circuits. Then point make circuits-build and sdk/web/scripts/build.sh at the wrapper.

CI strategy: Add a circuits job in ci that runs the builder once which uploads target/circuits-artifacts/ and the manifest. This will be used by other jobs. For eg., built-and-test.yml becomes:

jobs:
  circuits:
    uses: ./.github/workflows/circuits.yml # circuit build job

  build:
    needs: circuits # waits for circuits job
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
      - uses: Swatinem/rust-cache@v2
      - uses: actions/download-artifact@v4
        with: { name: circuits, path: target/circuits-artifacts }

      - run: cargo build --examples -p stellar-private-payments-sdk
      - run: BUILD_TESTS=1 cargo test --workspace --no-run
      - run: cargo test --workspace