Support additional_bindings

[mbarrien]

The parser and generator should add support for methods with additional_bindings, allowing multiple different URLs to access the same gRPC endpoint. The current code only renders code for the outer method, but silently ignores the additional_bindings.

This is especially useful when you have bindings using different HTTP verbs, like PUT and PATCH, that hit the same gRPC method but may use different verbs, or populate different fields from the URL.

[nicole-lihui]

I had the same problem 🙏