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.

Start by tracing how implicit routing handles google.api.http annotations and compare that behavior with AIP-4222, especially additional_bindings. Done means additional request parameters from those patterns are parsed and encoded with the documented separator behavior.

Written by the indexing model from the issue text.