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

Start with ci/cloudbuild/builds/generate-libraries.sh around lines 26-33 and inspect how it invokes the microgenerator through Bazel. Determine whether a genrule or custom rule fits; done means the script no longer carries the hard-coded invocation details and Bazel owns that generation step.

Escrito por el modelo de indexación a partir del texto del issue.