Respect `additional_bindings` in implicit routing

[dbolduc]

aip/4222 says:

If the google.api.http 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.

[coryan]

Strike one.

[scotthart]

This would involve parsing the additional bindings and if any variables are found add them (and their values) to the x-goog-request-params

https://google.aip.dev/client-libraries/4222#implicit-routing-headers-googleapihttp

[scotthart]

We still want to do this.