feat: mirror protoc releases to a Starlark constant

[alexeagle]

This allows rules_proto to supply a pre-built binary toolchain for protoc without needing any Bazel module supplied by the protocolbuffers maintainers. Obviates the need for protobuf to be released to the BCR as in https://github.com/protocolbuffers/protobuf/pull/16014 Allows the protocolbuffers team to stop caring about Bazel builds for external users, and to close associated bugs.

Next step in rules_proto supplying a toolchain that doesn't require users build protoc from sources.