To demonstrate how we can make our deployments more modular, this PR proposes archiving smart contract artifacts as tarballs that get uploaded to GCS. This  allows deployment tools to use precompiled artifacts rather than generating them on-the fly.

The archives are named after the hash of all Solidity files in the contracts-bedrock folder, including those in `lib/`, plus some additional metadata files like `foundry.toml` and `semver-lock.json`. See `calculate-checksum.sh` for details on how the algorithm works. I'm open to feedback around what should make up the checksum.

Since the atifacts are content-addressable, this PR also updates the CI pipeline to download the artifacts from GCS prior to running `pnpm monorepo`. When the Solidity codebase doesn't change, this allows the `pnpm monorepo` job to skip compiling Solidity altogether. While this won't work as well when we're actively modifying the Solidity codebase, since the hash will change, it does provide a modest speedup in CI.