Respect `additional_bindings` in implicit routing

aip/4222 says:

>If the [google.api.http](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto) annotation contains additional_bindings, these patterns must be parsed for additional request parameters. Fields not duplicated in the top-level (or additional_bindings) pattern must be included in request parameters, encoded in the same way.
>
> Much like URL parameters, if there is more than one key-value pair, the & character is used as the separator.

AFAICT, we ignore these.

Empieza rastreando cómo el enrutamiento implícito gestiona las anotaciones google.api.http y compara ese comportamiento con AIP-4222, especialmente additional_bindings. Se considera terminado cuando los parámetros de solicitud adicionales de esos patrones se analicen y codifiquen con el comportamiento documentado del separador.

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