Move microgenerator invocations to Bazel rules

[coryan]

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

google-cloud-cpp/ci/cloudbuild/builds/generate-libraries.sh

Lines 26 to 33 in 34c4b4c

	bazel run --action_env=GOOGLE_CLOUD_CPP_ENABLE_CLOG=yes \
	//generator:google-cloud-cpp-codegen -- \
	--protobuf_proto_path="${bazel_output_base}/external/protobuf~/src" \
	--googleapis_proto_path="${bazel_output_base}/external/googleapis~" \
	--golden_proto_path="${PWD}" \
	--output_path="${PWD}" \
	--update_ci=false \
	--config_file="${PWD}/generator/integration_tests/golden_config.textproto"

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(). That returns a Label object with all kinds of goodies: https://bazel.build/rules/lib/builtins/Label.html