Avoid spurious warnings on `*.pb.h` files with MSVC

Files generated by Protobuf have warnings under MSVC. There is little we can do to fix the code to avoid warnings, and disabling the warnings for all the code would mask problems in *our* code.

We can disable warnings in headers with MSVC if the header is included as `#include ` as opposed to `#include "foo/bar.pb.h"`. We have managed to fix this for googleapis protos, and the compute protos, but the test protos in `google-cloud-cpp` are not.

If we move the protos to `protos/...` (as oppposed to `google/cloud/...` or `generator/...`) then we can tweak Bazel and CMake to support include via angle brackets, then the warnings go away, and then we can start using newer versions of Protobuf with MSVC.

Localiza los protos de prueba en las rutas actuales de google/cloud o generator y compara sus reglas de compilación con las de los protos de googleapis y compute ya corregidos. Inspecciona primero la configuración relacionada de Bazel y CMake. Se considera terminado cuando los encabezados generados para las pruebas admitan la inclusión con corchetes angulares y dejen de producir advertencias espurias de MSVC.

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