Define a protobuf toolchain

[aherrmann]

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.