Bazel toolchain for pre-built protoc

protoc has always been distributed as pre-built binaries on https://github.com/protocolbuffers/protobuf/releases

Bazel 7 introduced --incompatible_enable_proto_toolchain_resolution to allow us fetch that binary rather than re-build it!

Why a separate Bazel module?

This belongs in rules_proto, see

Getting reviews from Googlers is hard so let's not wait for them.

Make sure your .bazelrc contains

# Introduced in Bazel 7.
common --incompatible_enable_proto_toolchain_resolution

Follow instructions from the release you wish to use: https://github.com/alexeagle/rules_protoc/releases