Support --incompatible_enable_proto_toolchain_resolution

[alexeagle]

Bazel 7 introduces this flag, allowing users to avoid compiling protoc from sources if they desire.

When this flag is enabled, rules like py_proto_library should use Bazel's toolchain resolver to locate protoc rather than use the hard-coded //:protoc cc_binary target in this repo.

As a concrete example, 8257c44 was performed before bazelbuild/rules_python#1577 landed, so even with the toolchain resolution flag enabled, users get a build failure if their CC toolchain isn't functional.

What version of protobuf and what language are you using?
Version: latest (27.1)
Language: N/A

[alexeagle]

It's possible this problem was only in py_proto_library. I don't know of other rules that exhibit this bug right now. So I'll close this with a plan to open a separate issue if users report more.