Define a protobuf toolchain

https://github.com/facebookincubator/buck2/pull/28 introduces a custom rule to download a protobuf distribution appropriate for the current platform and ready for use by rules like `rust_protobuf_library`.
This concept could be extended to promote protobuf to a proper toolchain under `prelude//toolchains` similar to e.g. `prelude//toolchains/cxx/zig`.
Before implementing this we should research protobuf toolchains in the Bazel ecosystem and see which lessons can be drawn for Buck2. A useful resource may be [this document by the Bazel rule author SIG](https://bazel-contrib.github.io/SIG-rules-authors/proto-grpc.html).