Migrate examples to Bzlmod once buildtools module is available

[limdor]

Most of the examples use third party dependencies only for the buildifier rule. In order to have it reproducible, we ping the version of rules_go, skylib, protobuf, and buildtools. With Bzlmod it would be much simpler because then we only pin builtools and we check in the log file.

Unfortunately buildtools does not have yet a module. Once it is available, examples should be migrated. https://github.com/bazelbuild/bazel-central-registry/issues/380

The only example that should not be migrated is the one that shows how to deal with third party dependencies without Bzlmod