Add Bazel attribute that generates a GAPIC using snake_case identifiers

[aohren]

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 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.

[summer-ji-eng]

@aohren want to follow up do you still need this feature. Let's sync.

[aohren]

Yea this is still something we'd like to have. I set up some time for us to chat about it.

[summer-ji-eng]

Customer fork the gax-nodejs to customized to keep the snake_case.(see change)