Move microgenerator invocations to Bazel rules

We use scripts to invoke the microgenerator via Bazel, for example:

https://github.com/googleapis/google-cloud-cpp/blob/34c4b4c3f61f716cb993c56dfd72486f427f1073/ci/cloudbuild/builds/generate-libraries.sh#L26-L33

It seems that kind of code really belongs in some kind of `genrule()` or a custom rule. There are many hard-coded magic strings in that call that can be converted to [`Label()`](https://bazel.build/rules/lib/builtins/Label#Label). That returns a `Label` object with all kinds of goodies: https://bazel.build/rules/lib/builtins/Label.html