Add Bazel attribute that generates a GAPIC using snake_case identifiers

Some consumers of Node.js GAPICs prefer snake_case rather than camelCase in the developer surface (e.g., https://github.com/Opteo/google-ads-node). To support this, they're employing a custom fork of [gax-nodejs](https://github.com/googleapis/gax-nodejs) to modify the casing. Supporting this directly as a GAPIC config option on Bazel build rules will alleviate them from needing to hack gax-nodejs.